I just earned the Tis the Season winter bash hat for posting at Dec. 25 (while today is Dec. 24).
Tis the Season means).
This is by design.
As previously with the And I Feel Fine hat, there's a grace period of 12 hours, on both ends of the specified time interval, to make sure that nobody misses out just because of their timezone.
See these MSO posts for verification:
From the latter:
This behavior will also occur for the other hats that are date-sensitive. This included L'Chaim, and will include Tis the Season and the New Year's Eve Hat / Auld Lang Syne.
Tis the Season is just a quirky way of spelling "It is the Season", referring to the holiday season.
But 'tis my heart that loves what they despise,
Who in despite of view is pleased to dote;
— William Shakespeare, Sonnet CXLI