Time References

One thing that's always been awkward for me is describing the day or time at which something took place. I wasn't sure if it was ideal to say something like 明日は and go on with the sentence from there, but then that used the topic particle I was saving for something else. Genki explains that I don't need the は particle in this case, though.

昨日映画を見ました。
Yesterday I saw a movie.

今日友達に会う。
Today I'll meet with my friend.

明日学校に行く。
Tomorrow I'll go to school.

Those sentences are all valid without the は particle designating the day as the topic. I could add it to each of them to create the same set of sentences with equivalent meanings, but it's not required.

Now, if I want to give a specific day or time, rather than something relative to my current state (such as today or tomorrow), then I need to use the に particle with it.

火曜日に映画を見ました。
On Tuesday I saw a movie.

十一時半に友達に会う。
I'll meet with my friend at 11:30.

九月に日本に行く。
I'll go to Japan in September.

The に particle is not typically used with things like "in the morning" (朝) or "weekend" (週末), but it can be depending on personal preference. It's also not needed for things like 毎日 (expressing regular intervals) and いつ (when).