"Where" as a conjunction

Solution 1:

Where is a relative adverbial pronoun of location, and relative pronouns can introduce relative clauses. Normally, it refers back to a location; however, the place–time conceptual metaphor is extremely common in Indo-European languages, and where can also be used to refer to a time or occasion. It is correctly so used in your first example, as in we dissected frogs in this experiment, where in uses the same conceptual metaphor of place for time. But it does sound a little bit informal.

In your second example, where is slightly less normal, probably because (elliptical) that is normally used after specific words indicating a time, such as day, moment, year, etc. I think that or a void is certainly preferable to your where, but it is probably acceptable. You can usually use where in sentences where you could replace it with in which or at which, but that is not possible here.

As to the comma, I would say the same rules apply as to other relative clauses: use a comma with non-defining relative clauses, but no comma with defining ones.

Solution 2:

'Where' is correct in example 1, but not needed in 2) as the locker is the location. All that's needed for that is to say "...last day I'll be waiting...'