From Charlie X :
Kirk : "On Earth today, it's Thanksgiving. If the crew has to eat synthetic meat loaf, I want it to look like turkey."
From Dagger of the Mind :
Noel : "Dr. Helen Noel, Captain. We've met. Don't you remember - the science lab Christmas party?"
From Data's Day :
Data : "Besides the arrival of T'Pel, events today include four birthdays, two personal transfers, the Hindu Festival of Lights, two chess tournaments, a school play and four promotions. An ordinary day."
(The E-D crew experiences four promotions a day on average? So the entire crew gets promoted an average of once every eight months. Wow.)
Interestingly, on Enterprise they twice gave us episodes set right around christmas (Bound and Harbinger are both set on 27th December, exactly a year apart), and made no mention of it. Yet it's still celebrated in TOS. Guess Archer is a Mr Scrooge McGrumpypants.
Give a man a fire, and you keep him warm for a day. SET a man on fire, and you will keep him warm for the rest of his life...