I'll meet my friends or I will go to the movies this weekend. 

주말에는 친구를 만나거나 영화를 볼거예요

What confuses me here is the particle 는 at the end of the word 주말. I know how to use 는/은 but why is it (being) used with an adverb???

You can combine certain particles together actually :)

It serves the same function it normally would.

"As for this weekend, (I) will meet my friends or see a movie."

It can also be shortened to 엔.

I believe someone else can provide you with more examples then.

주말 is a noun meaning weekend. 

에 in this sentences show the time an action will occur.

는 just acts like it normally would. It's used to contrast things, specify, and say "as for..." or "when it comes to...".