What is the difference between 日曜日【にちようび】and 日曜【にちよう】?

Question

In all the japanese learning books, they teach you that xxxday ends with 曜日.

But sometimes I get to see xxx曜 only without the "Day" Kanji.

Can I shorten it when speaking too or is it only written ? And when can I shorten it ?

Answer 1

It is not clear cut, but my impression is that the tendency is that 日曜日 means a particular day that is Sunday whereas 日曜 means Sunday in general or series of Sundays.

日曜日に出掛けます。
'I will go out on next Sunday.'

日曜に出掛けます。
'I go out on Sundays.'

When it is clear that it is not about a particular day but is about a series of days, this contrast becomes clear.

* 日曜日洋画劇場

日曜洋画劇場

Answer 2

When you say

But sometimes I get to see xxx曜 only without the "Day" Kanji.

do you mean in your books? Or other places too?

I personally only use it in informal situations like speaking with/emailing friends. I can't say I've ever seen it written this way non-informally.