The Cinque Terre are best spot for a sort stay any time of the year. Weather is nice and climate is mild even during Winter time, especially in Monterosso. The coldest months are December, January and February, vene though temperature and rain are lately very unpredictable.
The hottest months are definitely July and August which are also the most crowded ones along with Yay, June and September, but, if you do not mind being among the thousand lovers of the Italian Riviera it is also the season with longest days and more chances for fun and activities.
From March, April and October are amazing you want to visit the Cinque Terre in a more relaxed way.