From Wintersday 2007:
Friday, December 21st, noon PST Start of the event, NPCs appear, including Wintersday Priests, all quest givers except Rift Wardens and some collectors.
Saturday, December 22nd, ~4 AM PST Additional NPCs appear, including more collectors.
Saturday, December 22nd, ~2 PM PST Additional NPCs appear, including Candysmiths, Rift Wardens and Wintersday Gift Givers.
Monday, December 24th Additional NPCs, including Grentches, appear, presents start spawning.
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It's a safe bet that the same thing will happen this year. Since so far that has been spot on.