Scarlet Sails (Aliye Parusa) is a festival celebrating kids' graduation from high school, and while there are fireworks on the river at night, a concert with all the famous Russian pop stars you've never heard of, a tall ship with scarlet sails on the river and a general feeling of "celebration" throughout the day/evening, my experience was that it's pretty much just lots and lots of high school kids walking around the center getting drunk. If you don't mind crowds and over-excited teens, then it could be interesting to see.
It also depends on what kind of tour you're looking at though - many people love experiencing authentic local life when they visit, rather than just seeing the main tourist spots (in many ways this describes me, in fact). If that's you - then Aliye Parusa might be cool and shouldn't interfere with your sightseeing plans other than the one day it lasts. If, on the other hand, you've got a serious schedule planned, you might find walking around the center on that one day problematic as the day moves on towards evening. This too is easily solved, however, by planning to be somewhere else that day, if you choose not to experience the festival - for example Peterhoff, Oranienbaum, Pushkin or Pavlovsk.