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There is a sentence in a worksheet that goes: どようびは ほんを よみます。Why is it not
どようびに? When do I know which one to use and what is the difference?

Spoke to my sensei and she said you can use something like: どようびには, which
would still be right ... I didn't quite get what she meant. Can someone please
clarify, thank you.
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question
particles
When do you use は/に for どようびは/に?
The main difference is probably the nuance of the sentence.

は is used as emphasis on どようび as the topic of the sentence. "On _Saturdays_ I
will read books." (Only Saturday, and not any other day of the week.) It may
imply that reading books on Thursday is a habit of the speaker.

には is acceptable as well and means the same thing because は already indicates
emphasis on どようび. に helps to clarify that it is when the action takes place,
but it is quite clear even without it that どようび indicates the time of action.

If only に is used, however, it means that "On Saturday I will read books."
(Presumably the coming Saturday.) The action, _to read books_ , and not when
it is done, is emphasized. It does not imply that it is a habit or routine, it
just so happens that the speaker plans to read books on a Saturday.
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answer
With most time references, you can choose to omit に.

>   1. [土曜日]{どようび}[本]{ほん}を[読]{よ}みます。  
>
>   2. 土曜日に本を読みます。  
>
>   3. 土曜日には本を読みます。  
>
>   4. 土曜日は本を読みます。
>

are all grammatical sentences and roughly mean the same thing. 1 and 4 have
chosen to omit に. 3 and 4 make Saturday the topic of the sentence, when は is
thought of the topic marker, but は could also be the contrast particle, as in

> [今日]{きょう}[勉強]{べんきょう}しますか?  
>  今日は勉強する[暇]{ひま}がないんです。土曜日_は_本を読みます。
>
> Are you going to study today?  
>  Today I don't have time for studying, _but_ on Saturday I will read the
> book.

Similarly,

> [来週]{らいしゅう}は[京都]{きょうと}へ行きます。  
>  Next week I will go to Kyoto.

would be a sentence analogous to your sentence, where 来週 is the time
reference, which (as time reference) can be strengthened with に, or (simply as
part of the sentence) can be made the topic with は.
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> どようびは ほんを よみます。 -> I read books on Saturdays OR I will read a book on
> Saturday.

は, as the topic marker, can be used to refer to Saturdays in general or the
next Saturday from the time the sentence is spoken, in which case it indicates
a point in time without specifically having that function.

> どようびに ほんを よみます。 -> I will read a book on Saturday.

に marks the Saturday you are talking about as a specific point in time.
Without context this means the next Saturday from the time this is spoken.

> どようびには ほんを よみます。 -> I will read a book on Saturday.

When には is used, は directs the listeners attention to the specific time
indicated by に as opposed to simply stating a specific time. **It is a matter
of the feeling/nuance delivered to the listener**. In this case, it sets
Saturday apart from other days of the week and (according to a native
speaker※) gives the listener the impression that this content is something
that you haven't been doing thus far. So the entire meaning rendered in
English would be something like

> "I will read a book on Saturday which is something I have not been doing
> thus far and may or may not do again."

Also, remember that the option of どようび、ほんを よみます is an omission of particles,
making it more vague as it leaves the interpretation up to the listener. In a
general context this usually results in a straight transmission of
information, which is probably why it is used so much.

※I think it is safe to say that different people would get different
impressions depending on the context of the conversation. My goal in
explaining what a native speaker told me was to give an idea of the kind of
nuance that can be conveyed and not to say that this is the only meaning
associated with this usage. Also, in reality there are myriad ways to get は by
itself to refer to a specific point in time. For example, こんどの どようびは ほんを よみます。
-> I will read a book this Saturday. And as with any structure, に、は、and には can
usually all fit grammatically and all three will convey the correct
information. The difference is in the feeling and nuance conveyed.

Here is a Japanese website for Japanese people studying to be Japanese
language teachers that I based part of my answer on.
[助詞「に」と「には」の違い](http://nihon5ch.net/contents/bbs-study/old/mie-bbs.cgi?s=137)
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answer