A sleepover is a type of party when several young people stay for the night at a friend's home.

Question

(What is the part of speech of 'when'? Is 'when' a conjunction or a relative adverb? Can we use 'where' instead of 'when'?)

Answer 1

It's a subordinating conjunction in this sentence.
http://grammar.ccc.commnet.edu/grammar/conjunctions.htm#subordinating_conjunctions

It's better not to use "where" here because you're not referring to a place; you're referring to more of a time.