Will song of the welkin moon. Disappear??

So I recently opened my genshin account after 4 years and the banner, there was a time limit on the banner of the postlude.My question is will it disappear if don't complete in time ...I mean not reward but the postlude. Would I not be able to play it if the time limit is reached??

And can I play the song of welkin moon the main chapters of storyline if I don't do the time limited quest ??