Most grammer handbooks note that the comma before the conjunction is optional. The best way to see the country, unless you are pressed for time, is to travel on foot. Enclose parenthetic expressions between commas. A parenthetic expression is part of the sentence that could be omitted without changing the meaning of the sentence or making the sentence ambiguous. If the expression is a necessary part of the sentence, don't set it off with commas.
Dates usually contain parenthetic words or numbers.
4. letters, packages, etc. The abbreviations etc., i.e., e.g., the abbreviations for academic degrees, and titles that follow a name are parenthetic. If a term is restrictive, it cannot be omitted from the sentence. Restrictive and nonrestrictive clauses define the term which precedes them. People sitting in the back couldn't hear.