- Halloween:
Braaaaaains
- April Fools Day:
Yahooooooo!
- Nyan cat anniversary (of it being uploaded to YouTube):
~~~~~=[,,_,,]:3
- May the Fourth:
Luuuuuuke!
- Towel Day:
Foooooorty two
- June 4th (??):
KHAAAAAAAAN!
- Aaron will be late day:
Aaaaaaaaron!
- Pirate day:
Yaaaaaaaarr!
Obviously Google-specific in-jokes are probably less awesome though, so find some more globally recognised ones to replace those.