Now is morning
 

Can we say?:

-Now is morning/night/afternoon/evening.

NOW as a noun at the beginning does not sound very good. What is the correct way to express the meaning?

Indeed, in the examples you've written, using 'now' as a noun sounds strange.

I would be tempted to rewrite, although the use of the noun form may disappear.
Now it's morning.
It's morning now.
It's now morning.
It's morning.

We're more accustomed to phrases like:
Now is the time to ...
Now would be a good time ...

Thank you~

For other words, is THE optional in this case?:

-It's (the) afternoon/evening/night now. (I think AFTERNOON & EVENING sound good, but not NIGHT here, apart from the THE problem.

Adding 'the' is rare, actually, when you're speaking about 'now'.

It's afternoon right now.
It's in the afternoon right now.
It's the afternoon right now. (This can be said to convince another that it is no longer morning.)

It's evening right now.
It's evening time right now.
It's in the evening right now.
It's the evening right now. (Same comment as above.)

It's night right now.
It's nighttime right now.
It's the night right now. (This can be said, but used to distinguish one night over another.)

Thank you~

Can I say this?:

-Next night/evening he waited in the same place.
-The following night/evening he waited in the same place.

In English, NEXT DAY/MORNING sound good, but not NIGHT/EVENING.

The next night/evening ...
The next day/morning ...

The definite article is needed.

Thank you. Even after consulting dictionaries, my doubts are still unsolved. Are the plural and THE optional?:

-Now is (the) summer holiday(s)/vacation(s).
-It is (the) summer holiday(s)/vacation(s) now.

The article is needed.
In American English, we use 'vacation', the plural form reserved for back-to-back vacations. The summer vacation, though weeks in length, is one vacation.
The plural 'holidays' is chiefly British English usage. They would say 'summer holidays'.

Thank you~You say NOW cannot be used to tell the time in that way; then can I use NEXT WEEK/YEAR as a subject? No dictionaries have such sentences:

-(The) Next week will be August.
-(The) Next year will be 2027.