I am reading chapter 4 from the JLPT N5 book, where I found the following sentence:
After 昨日, why wasn't there a topic particle? Is "昨日は勉強しましたか。" an incorrect sentence?
Marking it with は is certainly not wrong, and would in fact be natural. It doesn't change the meaning, but if the は is stressed, it will contrast yesterday to all other days, and will thus put stress on the time:
昨日勉強しましたか - Did you study yesterday? (Plain question)
昨日は勉強しましたか - Did you study yesterday? (As opposed to any other day)
However, if the は is not stressed, then は is a normal topic marker and it is still a plain question:
昨日は勉強しましたか - Did you study yesterday? (Plain question, は unstressed)
(Thanks to naruto for pointing this out in the comments.)