The British-English phrase every Preston ¹ Guild, and its variants, mean very rarely.—Synonym: once in a blue moon.

¹ Preston is the administrative centre of Lancashire, a county of north-western England, on the Irish Sea.

For example, the following is from Sven’s massive strides at City suggest Capello could be facing uphill struggle, by Brian Viner, published in The Independent (London, England) of Saturday 22^nd December 2007—the Italian former football player Fabio Capello (born 1946) had just been appointed as manager of the England national team:

[Capello] is his own man, he does not court friendships or suffer fools. He will not be in thrall to the celebrity of a David Beckham or a Wayne Rooney. But never mind all that; has he the temperament to cope with seeing matches week in, week out, but his players only once every Preston Guild? Has he the guile to pick and organise a world-beating team consisting entirely of British passport holders? I hope so, but I don’t know so, any more than he or anyone else does.

The phrase every Preston Guild refers to the fact that Preston Guilds are held only once every twenty years—as explained by Edward Baines (1774-1848) in History, Directory, and Gazetteer, of the County Palatine of Lancaster (Liverpool, Lancashire: Published by William Wales & Co., 1825):

Preston Guild.
One of the most ancient and certainly one of the most splendid and elegant festivals in this kingdom is held in the borough of Preston every twenty years, under the designation of Preston Guild. It has been already observed, that these institutions are of Saxon origin, and Camden ² describes the Guilda Mercatoria, or Guild Merchant as a liberty or privilege granted to merchants, whereby they are entitled to hold certain pleas of land and other possessions within their own precincts, and thereby neighbours enter into associations, and become bound to each other to bring forth him who commits any crime, or to make satisfaction to the injured party. […] It does not appear that the institution of the Guild at Preston has ever been applied to these purposes; but whatever may have been the case before the date of the records that have come down to us, at present its ostensible objects are to receive and register the claims of persons having any right to the freedom or other franchises of the borough, whether by ancestry, prescription, or purchase, and to celebrate a periodical jubilee rendered distinguished by its unfrequent recurrence. The Guild commences on the Monday next after the feast of the decollation of St. John the Baptist, which is always in the early part of the autumn. The first Preston Guild on record was celebrated in the reign of Edward III. one of the royal benefactors of the borough […].
[…]
It is erroneously supposed by some to be obligatory upon the corporation to celebrate a guild every twenty years, on pain of forfeiting their charter, but no such obligation exists; the guilds have indeed, for upwards of two centuries and a half, been held at regular intervals, in virtue of a bye-law of the mayor, stewards, and aldermen of the guild, passed in the reign of Queen Elizabeth, but this is quite a matter of choice and arrangement, and should the entertainments and processions of the guilds ever wholly cease, (which goodness forefend!) no privilege or franchise would be lost by their discontinuance.

² William Camden (1551-1623) was an English antiquarian, historian and topographer.

These are, in chronological order, the earliest occurrences of the phrase every Preston Guild, and of its variants, that I have found:

1-: From Stage Land, by ‘Dangle’, published in The Clarion (London, England) of Saturday 23^rd January 1892:

It may be a bold thing to say, but I think—mind, I only say I think—that a little novelty in our pantomimes would not detrimentally affect the British Constitution. The British people, I know, are Conservative. They cherish the precious heritage won for them by the valour and sacrifice of their ancestors; they are proud of the grand old constitutional structures which antiquity has handed down to them; they reverence the fruits of tradition and precedent. Hence the hereditary form of government and the Pantomime wheeze. But as the former first-named institution was somewhat modified in the time of Charles I., and has still flourished since, we might perhaps—observe that I say perhaps—we might perhaps begin now (say at the rate of one every Preston Guild) to introduce two or three new ideas into our pantomimes. What do you think?

2-: From The Wigan Observer, and District Advertiser (Wigan, Lancashire, England) of Saturday 26^th November 1892:

CORRESPONDENCE.
PIT SERVICE AT ST. JOHN’S, PEMBERTON.
To the Editor of the Wigan Observer.

At last! there has risen one out of Israel. The time was when silence was golden, but I must endeavour to treat our quondam friend, Mr. J. Rigby, C.F.S.C.P.C., however much we may differ, as an honourable opponent, as he has not followed the example of some creeping vipers, but has signed his name to the letter. Mr. Rigby’s reference to the church parade in May, 1888, in commemoration of that great historical event, the defeat of the Spanish Armada, and in which I had the honour to take part, and which I venture to hope Mr. Rigby sympathises with, has no analogy with the service I alluded to. […]
The object of the united church parade had my entire sympathy, but the midnight service I denounce, and I shall be pleased to see Mr. Rigby cast-off that toadyism, and not be afraid to speak out what I know he thinks. I stated that the bubble was pricked, I might have said blown up. After the first service the whole thing collapsed, but, if rumour is correct, another attempt is to be tried. We shall see what we shall see. The next pit service is to be once per month, why not every Preston guild; that would be often enough. Hoping you will pardon me trespassing on your space,
I remain, yours truly,
W. L. White.

3-: From Pastimes, by ‘Muff’, published in The Clarion (London, England) of Saturday 12^th January 1895:

The third of the five test matches between Stoddart’s team and Australia commences to-day (Friday) at Adelaide. […]
[…]
It is quite evident the Australians have a big task in front of them to prove their superiority over the Englishmen in the test matches. […]
In times gone by Massie, Murdoch, Horan, Midwinter, McDonnell, Alec and Charlie Bannerman, Bonnor, Palmer, Spofforth ³, Boyle, Giffen, Blackham, Turner, and Lyons have raised our enthusiasm by their exposition of cricket, and though, with the exception of Giffen, Blackham, Turner, and Lyons, they are lost to Australian cricket so far as the representative matches are concerned, the colony is still prolific in good men, as testified by the deeds of Gregory, Trott, Darling, Iredale, Reedman, Coningham, Jarvis, Graham, Bruce, Trumble and the two McLeods. Considering that first-class cricket in Australia is comparatively of recent growth, the stride it has made is enormous, and speaks volumes for the earnestness and enthusiasm of the Cornstalks. Their present players are worthy successors to the old hands, who once or twice “spread dismay around” English cricket grounds.
True, they have not yet unearthed another Spofforth, but the “Demon” represented a class of bowlers a long way removed from the mushroom species, as one of his order only comes up about once every Preston Guild.

³ The Australian cricketer Frederick Robert Spofforth (1853-1926) was nicknamed the Demon Bowler.

4-: From the column Mirth in the Mill, published in The Cotton Factory Times (Manchester, Lancashire, England) of Friday 28^th June 1918:

“Some” Lie.
At a mill in Blackburn two spinners were arguing the point as to who had spun the finest counts.
Says No. 1: “I onest worked at a mill and spun counts up to 210’s. Heaw’s that for fine spinning?”
“That’s nowt. I onest worked at a mill where they spun counts thad fine we only doffed once every Preston Guild!” calmly says No. 2.
G. W., Blackburn.

5-: From the Fleetwood Chronicle (Fleetwood, Lancashire, England) of Friday 8^th September 1922:

Our Query Column.
What the Light-Keeper Wants to Know.

[…]
Who is the Knott End resident who has promised a new cap for the ferry engineers “every Preston Guild?”

6-: From this advertisement for Kendal Milne & Co., Deansgate, Manchester, published in The Manchester Guardian (Manchester, Lancashire, England) of Saturday 16^th September 1922:

KENDAL MILNE & CO.
purchase important Exhibit of Cotton Goods from
PRESTON GUILD 1922

Twenty years ago, on the occasion of the Preston Guild of 1902, we purchased the complete Exhibit of Cotton Goods manufactured by

HORROCKSES, CREWDSON & CO.,

and the subsequent sale of these goods was an event which is remembered even to-day by many Customers.
The 1922 Exhibit by this famous firm has also been secured by Kendals, and the beautiful examples of Cotton Goods are now offered to Customers at

SPECIAL LOW PRICES.

All this merchandise has been specially manufactured by Horrockses for the wonderful event just concluded, and it is not possible to produce better goods or higher quality—it is an opportunity such as only occurs “every Preston Guild.”