Reasonably priced hotels for impromptu trips to Austin are darn near impossible to get in February (Austin Marathon), March (South by Southwest), during UT football season, and during the Austin City Limits Music Festival (October) unless you plan far in advance (at least six months). It’s also a good idea to make reservations for cars and popular restaurants, like Uchi, Driskill Grill and others as soon as you know you’re coming into town.
Austin weather gets really hot here in the summer, which makes for a pretty miserable time of walking around and exploring the sights unless you just sit in Barton Springs all day. For that reason, June through August is not the best time to visit Austin. If you must visit during the summer, plan your trip in June, when it’s not (usually) as hot. December and January run into the holidays, so you can usually find some good hotel deals and packages. Really. it's true.
Early spring (well after South by Southwest in March) is one of the best times to visit Austin. Hotel prices are more reasonable, all the SXSW out-of-towners have mostly left, and the bars and restaurants aren’t nearly as crowded. And it’s not blazing hot yet—a plus.