This depends on weeks. First part of July has July 4 crowds. Two weeks around Easter have the Easter crowds. Most crowded weeks of the year are in order, Christmas week, the two Easter weeks, and July 4 time (from few days before to few days after). For those times, if you have never been, just imagine that the crowd is larger by likely double or more than anything you have ever seen at WDW.
Outside of that the other summer weeks from mid-June to mid-August are somewhat more crowded than the other spring break weeks, although those also have large corwds, and all the other spring break and summer weeks are noticeably less crowded than the big four mentioned above.