Sentence 1a: You can come and stay with us whenever you want.
Sentence 1b: You can come and stay with us no matter when you want.
Sentence 2a: You can ask for help whenever you need it.
Sentence 2b: You can ask for help no matter when you need it.
Does sentence 1a have the same meaning as sentence 1b? Does sentence 2a have the same meaning as sentence 2b? In these situations, Do “whenever” and "no mater when" introduce concessive clauses?
To begin with, all the four statements mentioned above are indicative of very friendly and generous offer. Second, there is a subtle difference of meaning when we use "whenever and no matter when". I take the sentences 1b and 2b a step further compared to 1a and 2a. "Whenever" means at any time, whereas the phrase "no matter when" is used to mean regardless of any situation or time or whatever the time or situation is or it will cause no problem to the speaker. So I think "no matter when" is more emphatic and generous in offering the help.
As for concessive clauses, these sentences don't introduce such clauses. Concessive clause is a clause (usually beginning with although, even though) expressing an idea that suggests the opposite of the main part of the sentence. In these sentences, there is nothing like that. However, if we think otherwise and are sceptical about the speaker's bona fides in regard to his offer, it is another thing.
I would say that in each case, the two versions are basically synonymous, but with the no matter when version being much more forceful.
"You can ask for help whenever you need it" is friendly and polite; but later, when you wake up the speaker at 2:00 AM to ask for the help, you might find that it wasn't meant to be taken quite literally.
"You can ask for help no matter when you need it" sounds like a response to some implication that there may be some times when the interlocutor can't ask for help; for example, I can imagine his/her having just said "I know you said I could ask you for help, but I wasn't sure if you'd want to be woken up at 2:00 AM."
