Miami and Miami Beach are year-round destinations. Most visitors come October through April, when the weather is close to perfect; hotels, restaurants, and attractions are busiest; and each weekend holds a festival or event. "Season" kicks off in December with Art Basel Miami Beach, and hotel rates don't come down until after the college kids have left from spring break.
It's hot and steamy from May through September, but nighttime temperatures are usually pleasant. Also, summer is a good time for the budget traveler. Many hotels lower their rates considerably, and many restaurants offer discounts—especially during Miami Spice in August, when slews of top restaurants offer special tasting menus at a steep discount (sometimes Spice runs for two months, check www.miamirestaurantmonth.com for details).