sex vs. chocolate
The two words have a roughly comparable baseline frequency, with 'sex' always getting just a little more talk-time than 'chocolate'. There's a tiny peak where 'chocolate' rises above 'sex' on Hallowe'en, a couple of similarly tiny ones throughout advent, a bigger one at Christmas, and then a huge one on Valentine's Day and an absolutely massive one at Easter.
Interestingly, there's no more talk of sex than usual on Valentine's Day, and a notable dip at Christmas. Guess no-one's in the mood after all those big dinners.
ETA: Also amusing is the pronounced dip in the use of the word 'birthday' on the last day of February this year.