
LEONARD: I can't believe you think
he only wears it on Halloween.
The Big Bang Theory Season 4, Episode 24

I dressed up as
him for Halloween.
The Big Bang Theory Season 5, Episode 21

He didn't remember me, because the last
time we met I was in a Halloween costume.
The Big Bang Theory Season 2, Episode 14

Happy Halloween.
The Big Bang Theory Season 1, Episode 6

All Hallows' Eve--
thus, Halloween.
The Big Bang Theory Season 12, Episode 6