This website tells you if its Christmas. Its accurate: 96.7741935% of the time!
EDIT: @joshwood pointed out its 99.726027398% accurate
EDIT 2: @joshwood pointed out again its: 99.726214921% accurate!
EDIt 3: Thanks to (once again) @joshwood, theres a file for how accurate it is!
EDIT 4: @EthanCulp changed it again. Check file.
edit 5: no change
edit 6: will give cycles for more decimals
edit 8: @DJWang gave more decimals