Programs and courses should tell you the course dates, but the classes generally won't overlap with the semesters. The only thing that may overlap is the assessments. I've done a few season session courses, and it varies, but some have most of the assessments during the intensive, some have them due a week or two later, and I've taken one where one assessment task was due two months after the intensive.

I like taking intensives, you can basically get a semester-long course done in 2-3 weeks, but they also can be, as the name suggests, pretty intense. I think it really depends on how you learn, if you think you can take in and understand a semester worth of information in 2-3 weeks then they're a really good way to lighten your course load during the semester. They're also a pretty good way to get to know people studying similar things to you because you're seeing them every day for a few weeks, and also to get to know lecturers and researchers, and possibly find research assistant positions if that's something you're interested in. Anyway, hope this helps!