What episode of alf has the happy anniversery song?

The episode of ALF where the "Happy Anniversary" song is featured is called "The Birthday Party" (Season 3, Episode 2).

This episode features ALF's attempt to throw a surprise anniversary party for his human friends, Willie and Kate, which hilariously goes awry. The song is sung by ALF himself, with lyrics that are a bit off-key and awkward, but ultimately endearing.