I wouldn't put it in this sentence strucutre ,but as Denis mentioned it depends whether you want to lay stress on the clause or not.

Elizabeth, Putting a comma before 'as' is not necessary in your sentence. It's a great day as I've helped someone. On the other hand it is not wrong. It is a matter of writing style. It's a great day, as I've helped someone. The comma emphasizes the clause (as I've helped someone.) If that is your intention, then the comma is perfectly okay.