Sitges is at its best in May, June, and September — warm enough to swim, long days, fewer crowds. July and August fill up with day-trippers and holiday-home owners; the town doubles in population and the seafront restaurants run at full tilt.
October has the Film Festival and surprisingly warm sea water. February has Carnival, which is unmissable but turns the town into one big party for a week — book hotels months ahead. Winter outside Carnival is mild, sunny, and very quiet, with many seafront places shut.
Avoid the second week of August if you can. The Spanish national holiday on the 15th sees the town at its absolute fullest.