Something fancy to look at (maps covered in snow on Christmas or pumpkin piles on Halloween)
Thematic paint jobs, decorative consumables (“limited edition” vanity items)
New event-specific activities
Special deals/sales
More attention to national and religious holidays
Anything would suffice. Just give us some holiday spirit!
Holidays don’t matter at all. I’ll just play as I usually do
Special effects and items (fireworks, Easter bunnies, snow)