Adverb versus instrumental case in time expressions

Question

In time expressions, I'm confused whether I'm looking at an adverb or a noun in instrumental case.

For example, летом can be an adverb meaning in summer, or the instrumental case of лето (summer).

And вечером can be an adverb meaning in the evening, or the instrumental case of вечер (evening).

Now take these two sentences:

1) Может быть, они приедут к нам летом.

2) Мы ждём гостей в субботу вечером.

What is the form of летом and вечером?

Answer 1

Those are adverbs.

Try replacing the words with a question:

Может быть, они приедут к нам чем?

Мы ждём гостей в субботу чем?

This makes no sense.

However, in a more complex example:

Может быть, они приедут к нам жарким летом

, жарким летом is an adverbial phrase (answering the question когда?) which consists of a noun and an adjective.

Answer 2

As stated Quassnoi, those are adverbs, and question "чем?" makes no sense. No sense - today. This is by definition of adverbs. But how was adverbs appeared long time ago? There were some meaning words , which was stripped. This is implication. I attempt to reconstruct such words: For example. Может быть, они приедут к нам тогда, когда пора года явится (чем?) летом. Or: ... тогда, когда жизнь природы обернётся\воспринимается (чем?) летом. Or: ... тогда, когда пора воспринимается (чем?) летом\вечером. Thought, it is semanticly correct, but no one say it.

Sorry for my english.