"It will snow this evening."
Snow is the subject, not the object. And general time references like 今日 or 今晩 don't generally require a は after them (though you can usually insert one to emphasize the time as the topic of the sentence).
As for the ga and Wo, GA is a subject indicator WO is a direct object indicator. That's all I know...
"Last night" gets a は in "it snowed last night", but tonight doesn't get one. Why?
は is optional after relative time words like tonight, yesterday, last week.
I think that including the topic marker puts more emphasis on the time part of the sentence.
This is correct - it depends on whether the speaker wants to emphasize it.
I see! What was confusing about this question was that I didn't realize 'こんばん' was assumed to have a comma/pause after it, which is totally invisible when you answer it here. With no comma, I felt like I needed the は for correct grammar, but it makes sense now.
^ im confused on that too. When you say Konbanwa it means "good evening" but I would imagine you'd be saying it while its already evening similar to saying good morning. So if evening is a time of day you can be in, why is evening supposed to be some expression of the future, or can it be both?