The past couple of years Amsterdam has developed a huge festival scene; I dare to say one of the biggest in the world. With approximately 400 festivals each year (in Amsterdam alone!!) there is a festival for anyone, at any time. The festivals vary from art festivals to food festivals and most famous: the dance festivals, variating from small and indie to big and commercial. Dutch DJs are amongst the best in the world and known for their new sounds. Even though there are festivals all throughout the year, THE festival season starts in April when it is gets a bit warmer outside. During that time the first outdoor-festivals make their appearance and the last ones close the season at the end of September. Usually these festivals start at noon and end at 23:00 or 00:00, but there are always after party's in clubs down town.
You will be able to go to many big festivals with a huge line-up of well known, really good DJs, playing on 5 to 8 stages (Awakenings, Voltt and Welcome to the Future are well known examples), but you can also find many smaller festivals with one or two stages on which a couple of new talents are playing and sometimes one or two well known DJs (for example Dagdromen Festival or Betoncentrale). Tickets sell out very fast when the line-up is has some big names (sometimes months in advance) so check regularly and otherwise use Ticketswap.
Since a couple of years ear there is also a smallish (they grow fast!) Sunday festival where big names perform from time to time (ending at 20:00 or 22:00), on a city beach created in Amsterdam-West. This festival called Thuishaven (in the winter: Wintercircus) facilitates a Sunday-chill environment where you can finish your weekend with some dance moves - awesome!