Using "Whether" or "Whether or not"

Solution 1:

Bryan Garner, Garner's Modern American Usage, second edition (2003) answers this question in a consistent way that is easy to apply to particular instances of "whether (or not)":

Whether or not. Despite the superstition to the contrary, the words or not are usually superfluous, since whether implies or not—e.g.,:

• "In another essay, 'The Rules of the Game,' he discusses moral codes and whether or not {read whether} they work." Diane Hartman, "At Life's End Carl Sagan Awed by Life's Unknowns," Denver Post, 22 June 1997, at D6. [Other examples—involving "deciding whether (or not)" and "decide whether (or not)"—omitted.]

But the or not is necessary when whether or not means"regardless of whether" {the meeting will go on whether or not it rains}. [Other examples omitted.]

By Garner's reasoning, you don't need or not in this sentence:

You can decide whether to go once you've finished your homework.

but you do need it in your original sentence:

You can go whether or not you finished your homework.

Solution 2:

Usually when using the word whether in a sentence, the or not is superfluous (i.e., not required). However, when whether or not means regardless of whether, the or not part is required.

Examples of when or not is required:

• The postman will deliver my mail today whether it snows or not.
  • (…regardless of whether it snows.)
• The former CEO of Global Bank is going to jail today whether or not he is actually innocent.
  • (…regardless of whether he is actually innocent.)

Note: The or not does not have to follow immediately after whether, so there is some leniency on where it is placed.