は vs に Question (from Genki) So in the dialogue of Chapter 3 in Genki 1, there is the sentence "...週末はたいてい何をしますか。", and later on in the chapter there is the sentence "週末(に)何をしますか。" I understand that in the first sentence は is marking the topic and に in this case is denoting time, but seeing as both sentences are conveying the same idea (minus the frequency), is it OK to use either は or に in both of those sentences, or am I missing something subtle?