Wa particle confusion

In these 2 sentences (have to use romaji bc no JP keyboard):

ashita wa nihongo no kurasu ga arimasen

ashita kyouto de omatsuri ga arimasu

Why is there a wa particle in the first sentence but not in the second one? Would love a general explanation on how to use wa in these kind of sentences and when not to use wa.

If i remove the wa from the first sentence it would feel wrong bc I’m not marking tomorrow as the topic despite it being talked about so i guess it’s not removable?
What about the 2nd sentence it feels ok to add wa particle but there’s no wa particle in the og sentence so i wonder why.