The phrase coming? so is Christmas! is addressed to one who, saying coming! (i.e., in a minute), takes a long time to arrive. It is used by extension of anything that is being delayed.
For example, the following is from Disappointment that work on VEC 1 school extension has yet to start, published in The Argus (Dundalk, County Louth, Ireland) of Friday 24th December 2004:
Strong disappointment was voiced that a start as promised had not been made on the extension to Scoil Ui Mhuiri 2, Dunleer by the end of the year at the December meeting of Louth VEC.
Cllr. Tommy Reilly complained that all the committee received was promises and letters that it [= the extension] was coming. “So is Christmas,” he remarked.
1 VEC is the abbreviation of Vocational Education Committee, denoting a statutory local education body in the Republic of Ireland.
2 Scoil Uí Mhuirí is a multi-denominational post-primary school in Dunleer, County Louth.
The phrase is first recorded in A Complete Collection of Genteel and Ingenious Conversation, according to the Most Polite Mode and Method now used at Court, and in the Best Companies of England, by the Irish satirist, poet and Anglican cleric Jonathan Swift (1667-1745). Published in London in 1738 but composed in the first decade of the 18th century, this book is a satire on the use of clichés; its purported author, Simon Wagstaff, assures “the Reader, that there is not one single witty Phrase in this whole Collection, which hath not received the Stamp and Approbation of at least one hundred Years” (cf. in this regard the authentic origin of ‘to rain cats and dogs’):
[—Footman comes in.—
Lady Smart. Did you call Betty?
Footman. She’s coming, Madam.
Lady Smart. Coming! ay, so is Christmas.
The earliest occurrence of the phrase that I have found is from a letter to the Editor, published in The Sydney Herald (Sydney, New South Wales) of Monday 5th December 1836—The Australian was a newspaper published in Sydney:
I look upon the system of selling land as a good system, if the Government keep faith with the public, and appropriate every farthing of the money religiously, to the purpose for which it was raised, that of increasing Emigration […].
When I and others bought land years ago, I looked upon it as a sort of voluntary subscription for the increase of Emigration; but where are the Emigrants whose passage we all subscribed to pay for? The Australian says they are coming—so is Christmas; and my belief is, that the money is locked up in the Government coffers for purposes of patronage, and the settlers ought to remonstrate on such a shameful misappropriation of their money.
The second-earliest occurrence of the phrase that I have found is from the Wyoming Republican and Farmer’s Herald (Kingston, Pennsylvania) of Wednesday 26th April 1837:
Beauties of Spring.—On Sunday, April 23d, we had a fall of snow, commencing at about 9 o’clock in the morning, and lasting till late in the evening, which gave our mountains a mantle of nature’s purest white. Monday morning was ushered in with a North Wester, which would have done no disgrace to February.
“Spring-tide is coming! coming! coming!” So is Christmas.
The phrase then occurs in the proceedings of the Norfolk Quarter Sessions that “took place with reference to the “Hero of Redan,” Major-General Windham” 3—proceedings published in the Evening Mail (London, England) of Monday 22nd October 1855:
The Earl of Albemarle […] said […] it was the more imperative that he [= Windham] should receive the approbation of his countrymen, for, although 33 colonels were promoted and many of them received the Commandership of the Bath, he remained for nearly a twelvemonth unpromoted, undecorated, and unmentioned. There must be something rotten in the military system of this country that such merit remained so long neglected. (Hear, hear.) Sebastopol was now taken, and they naturally expected that the authorities would have thought proper to recommend Her Majesty to bestow some special honours upon General Windham. Those honours were coming, so was Christmas, and which would come first? The late distribution of honours reminded him (Lord Albemarle) of the strolling players who played the tragedy of Hamlet and omitted the part of Hamlet by particular desire; for the name of Charles Ash Windham, the only general who left the trenches on the 8th of September, was omitted from the list. (Laughter.)
3 The British Army officer Charles Ash Windham (1810-1870) had led the charge on the Great Redan to the south of the Malakoff redoubt at Sebastopol on 8th September 1855 during the Battle of the Great Redan in the Crimean War.
—Other posts referring to Christmas:
– an investigation into the Christmas Truce of 1914
– history of ‘Xmas’, abbreviated form of ‘Christmas’
– origin of ‘Boxing Day’ (the first weekday after Christmas Day)
– meaning and origin of the phrase ‘like turkeys voting for Christmas’
– history of the phrase ‘(all) dressed (up) like a Christmas tree’
– the phrase ‘what else did you get for Christmas?’
– ‘all one’s Christmases come at once’: early occurrences