Indicating time in Japanese: に、は、or no particle?


This is my first question on Japanese Stackexchange. よろしくお願いします！

The following example sentence made me wonder about the placement of particles
after time expresssions:

先生は週末は何をしますか。

I came up with this sentence myself. My friend (a Japanese learner just as
myself) thinks that the は after 週末 necessarily indicates contrast and he
thinks that 週末 can in fact take に. Without context, I think the contrast
function is a legitimate way to think about it は in this sentence. But I
assumed that 週末 always either takes the particle は or no particle at all,
similar to words like 今日 or 明日 (<http://www.punipunijapan.com/time-particle-
ni/> this website lists a few more words that describe time but which cannot
take the particle に).

**So here are my questions:**

1) Is there a systematic way to know whether a given time expression takes the
particle に or the particle は?

2) If a given time expression cannot take the particle に and は instead, are
the only options to use it in a sentence to either 2a) use は, which may result
in awkward sentences when theres another は closeby, or to 2b) use no particle
at all, oftentimes leaving a comma 、 after the time expression?

DISCLAIMER: I know that some time expresson can also take other particles,
such as 後で. I think there aren't so many of these and they don't cause any
confusion for me, so these don't need to be addressed imo.




The difference between 週末は/に/- is subtle. My take is the following,

> 週末は何をしますか？ and 週末、何をしますか？

It is asking what you are doing regularly on weekend or what you are doing on
weekend if you have free time then.

> 週末に何をしますか？

It is asking what you are doing especially on weekend or do you have some
particular things which you can do only on weekend.

今週末に何をしますか？ might imply there is an important event on this weekend. So, you
need to ask their schedule on this weekend to invite for that.

