There is no mention to 今天晚上 in the English sentence!

Is 一个 really necessary here? I feel like I wouldn't need it unless I want to emphasise that I'm only going to ONE party.

"I will go to a party. I need to dress up a bit". I don't know why i have to translate 令天晚上....

Why not “我会去派对，需要打扮一下” ?

Why not “我会去一个派对。我需要打扮一下” ？