A hyphen (-) is a single short mark. It is used to join words or numbers (see 3.1). Hyphens are used to connect compound modifiers, however, they are omitted when the first modifier is an adverb ending in -ly.
Right: Reunion Weekend is June 7-9.
Wrong: This mostly-ignored manual is of little use.
A m-dash (—) is a single long mark, not two hyphens (- -). (On a Mac, option-shift and minus/underline.) In print it is used without leading and following spaces.
Right: Reunion Weekend—an event for alumni—is June 7-9 (or “from June 7 to 9”).