You can have two nouns together with the first one acting as an adjective that modifies the second one. It's called an attributive noun.
So, is the first noun showing a possession or is it acting like an adjective? If the answer is the latter, you cannot replace it with a possessive form.
the morning newspaper (instead of "the morning's newspaper") = I believe "morning" here is an attributive noun. The version with an apostrophe sounds unnatural to me.
last Tuesday workshop (instead of "last Tuesday's workshop")
Last Monday meeting (same as above)
Is it "last (Tuesday workshop)" or "(last Tuesday) workshop". If the answer is the second, you need an apostrophe. This is the more likely meaning.
If you actually mean you had several Tuesday workshops and this is the last one of them, you don't need an apostrophe.
Same for the other example.
Student or Learner