The phrase all one’s Christmases come at once, and its variants, mean that one is experiencing remarkably good fortune, that one has everything one could have wished or hoped for.

This phrase seems to be of Australian-English origin. The earliest occurrences that I have found are:

1-: From The City at Easter Time, published in The Gundagai Independent (Gundagai, New South Wales) of Monday 4^th April 1932:

Well dressed men and prettily clad matrons and maids thronged the city streets during the Easter festivities. Outward appearances showed Sydney to be as busy as ever. Everyone seemed to have money, and people rushed to gain admission to amusement places.
[…]
The country folk carried a boom in railway activities. During Easter week the Central Station was crowded, and porters thought all their Christmases had come at once—they had, for ’tis certain the country people will not return for many moons.

2-: From The Longreach Leader (Longreach, Queensland) of Saturday 25^th January 1941 (in an extended form, with the addition of birthdays):

LOTS OF NEWS
From “Gay Lad,” Whitewood Siding, Winton Line

Dear Uncle Jim,—
How are you and everyone at the Den. I feel fit and fine for some more Christmas celebrations. We let off crackers on Christmas night, and as I was lighting one, dad put one under me and gee, it made me jump. I thought all my birthdays and Christmases had come at once. Mum threw one and as I was running, it came down and landed on my back and went off. I was kept busy running that night.

3-: From The Narandera Argus (Narandera, New South Wales) of Friday 14^th February 1941 (again with the addition of birthdays):

Letter From London
Former Narandera Resident’s Exciting Experience

Extracts from a letter written by Miss J. McPhee, late Matron of St. Faith’s Hospital, Narandera, to some Narandera friends, make very interesting reading. Miss McPhee is doing hospital work in London, and says:
[…]
“We had a 1000lb. high explosive direct on the house two doors away about four weeks ago, when I was doing a week’s night duty. […]
“None of the patients was injured and apart from the windows, kitchen wall and ceiling, and the shadowless lamp, there was little damage. I had a good few bruises and thought all my Christmases and birthdays had come at once, but otherwise I was little the worse for the experience, and after changing my uniform carried on for the remainder of the night.”

4-: From an article about “Sister Betty Jeffrey, of the Australian Army Nursing Service, a returned prisoner from Sumatra, who reached Melbourne last night”, published in The Herald (Melbourne, Victoria) of Thursday 25^th October 1945:

Last night, the Jeffrey family in Ailsa Avenue, East Malvern, listened to her until 1.30 a.m.
Then they lifted the dozens of presents from her bed, and she fell into it.
The house today looks as if all their Christmases had come at once. There are hundreds of telegrams, letters, presents, and flowers in all the rooms.

The phrase has come to be used in American English too—as in the following from The Windsor Daily Star (Windsor, Ontario, Canada) of Wednesday 16^th November 1955:

Halo! Halo! Everybody

The poor family on whom Humphrey Bogart, Aldo Ray and Peter Ustinov descended, thought all their Christmases had come at once.
The three actors, about as unseraphimic a trio as even Hollywood could drag up, take the lead roles in the current Tilovi hit, “We’re No Angels,” * set in the Yuletide on Devil’s Island.
The desperadoes, callous to the extreme, escape the sordid hell that is their prison and descend upon a poor trader’s family intent on fire, rape and pestilence to say the least. However, the sorry plight of the family brings out the fact that about the only callousness the convicts have is on the palms of their hands.

* We’re No Angels (1955) is a Christmas comedy film produced by Paramount Studios, directed by Michael Curtiz, starring Humphrey Bogart, Peter Ustinov, Aldo Ray and Joan Bennett.

The phrase has also come to be used in British and Irish English. This, for example, is from the Press and Journal (Aberdeen, Aberdeenshire, Scotland) of Saturday 21^st January 1989:

Every Wednesday morning, the Stronach Mother and Toddlers’ Group meet in one of the ante-rooms at the village hall, while the Stronach Crochet Circle meet in the other.
While Babbie Girn, Virginia Huffie and Geneva Brose meet in Room A to click their needles, Flo Spurtle, Claire Macfarlane and a dozen other young mums crowd into the smaller Room B.
With so many examples of Stronach pulchritude in the one building, hall caretaker and village sexist Erchie Sotter finds that all his Christmases have come at once.