There are a couple of problems with this whole thing.
1. Terra doesn't exactly have seasons any more. Ever since it was ravaged during the Horus Heresy, it's been an ecologically devastated planet-wide urban sprawl. Imagine Mumbai, times one hundred, everywhere, for ten thousand years. The only weather changes worth describing would be "smog" and "thick smog."
That, oddly, did not occur to me, which is sad since all three of you have commented on it. Ah, well...
2. It wouldn't matter anyway, since the Feast is celebrated on over a million Imperial worlds and according to the Law of Science Fiction Variance they're all going to have wildly different and thematically appropriate seasonal timetables. The Feast might happen in the dead of winter on Snowyworld, but it might also be right at the end of the fifteen-month summer on Sunnyworld.
Oh, I'm certain that that would be the case on a local level, but since the dating system the Imperium uses seems to cycle every 365 units, I guessed they were basing the official calendar on Terran years. So while you wouldn't have consistent weather, you'd still be able to say, "Hey kids! Just three more days until we get to burn the library!"
3. The holiday itself has little in common with either Easter or Christmas. Christmas is the day of Jesus' birth, and Easter is the day of his resurrection. Neither one is thematically similar to the Ascension, which was when the Emperor died and got put in the Golden Throne. The closest real-world equivalent you've got is when Jesus went to Heaven, which isn't actually celebrated anymore. And the date for that holiday, which is called Ascension Thursday, varies every year. It was the 21st May this year, and it'll be the 13th next year. It can be any day between April and June.
Perhaps my first post was unclear... I know that there's no thematic connection, but I picked those two points in the year because of our real-world tendency to celebrate at those times. Admittedly the climate changes may have made that moot, but a surprising amount of the past survives in the grim dark future, so I thought that tendency might as well.
Also, thank you for pointing out Ascension Thursday. Despite being raised Catholic, I had never heard of it, although I had wondered why that wasn't holiday-worthy. A nice bit of trivia, and for that you get another Rep Point to swim in.
4. Even if we pick the 25th of December and say that this is the Emperor's Ascension, how do we get every planet to celebrate it simultaneously? Even Earth doesn't celebrate Christmas all at the same time because of how the day works. It's far more likely that each planet has its own internal calender and each one picks an appropriate day for the Feast. In which case, tied with 2., it could happen any time.
It
would be a logistical nightmare, but that seems to be meat and drink to the Administratum; I can picture that department being one of their largest. Plus the Ecclesiarchy seems pretty anal retentive about tradition, so I could see proper celebration being enforced locally.
5. It doesn't even matter because celebrating Christmas in a fictional universe is almost always lame and kind of ham-fisted. Does anyone remember the Star Wars Holiday Special? Lifeday? Yeah. Not cool.
Missed that movie, but I'm glad to have done so. Anyway, I'm not trying to cram in a real, modern holiday, just thinking about the in-universe ones; partly for curiousity, and partly because I'm running a
Dark Heresy campaign and I like having living, breathing worlds with details like that.
So if I was going to pick Christmas or Easter I'd prefer to say none of the above and make up my own fictional holiday celebration. Knowing the Imperium, the Feast probably involves burning a year's worth of heretics and kicking some puppies, followed by the Ten Hour Hate where everyone boos a picture of Abaddon.
Oh, I'd imagine the Ten Hour Hate to be more of a monthly thing, really. Weekly on Cadia...
If you're curious, the Feast is described as follows:
Dark Heresy: the Inquisitor's Handbook; Bligh said:
This annual event is the most common festival held in the Emperor's name. It celebrates the moment, ten thousand years ago, when He was raised to the Golden Throne and made the transition to divinity. On many worlds this is a week-long event, with feasting, games, and spectacles to honour the Emperor. Often symbolic sacrifices are made in His name, such as the burning of books to offer their knowledge up to Him. It is also an auspicious time to join people in wedlock or concieve children, leading to mass gatherings of citizens intent on one or both of these undetakings. It is also said that during the Emperor's Ascension, no false judgements can be made, making it impossible to overturn a ruling made during this time.
Not all worlds celebrate the Ascencion with feasts and festivals. On some, especially primitive worlds, this holiday marks a time when the Emperor looks down from His divine realm and remembers His mortal followers. It is at this time that those brave and bold enough can eran a place at His side. Thus the Ascension becomes a bloody time when men and women fight and die in an effort to please the Emperor, each trying to outdo the courage and skill of the others.
However it is celebrated, the Fest of the Emperor's Ascension is the most notable Imperial holiday and the one that the majority of Imperial civilians observe, even if they do so only to gain a brief respite from their daily drudgery.