My understanding is that some of the time-words are marked by "wa" and some
are not.

Also there seems to be a nice rule of thumb where if a time word in English is
preceded by in/at/on etc. then the equivalent Japanese word takes "wa".

I have seen "kyou" used with and without "wa". Based on the rule of thumb
above , "kyou" should not take "wa" as the English word "today" does not take
in/at/on etc.

Maybe the standard is without "wa" but in everyday speech "wa" is accepted ?

Maybe with and without "wa" are both accepted ?

Maybe with and without "wa" carry a different nuance ?

Maybe with "wa" is more casual than without "wa" ?

I used "kyou" throughout for my question but the same question extends to
"kinou" and "ashita".

What is the rule regarding the use of "wa" with "kinou" , "kyou" and "ashita"
?

If anyone wants a specific example then how about ''It is hot today.''

今日暑いです or 今日は暑いです ?
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question
grammar
Is wa used with kyou ? 今日 or 今日は?
When you say 今日暑いです that would simply mean that it is hot today. But if you
say 今日は暑いです you are comparing today to some other day. So, it may mean it is
hot today, but it was not hot yesterday. The form without は does not have that
nuance.
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answer