Following the initial course registration process that happened recently, solar re-opens July 11th and then several times later in August, I think and I completely forgot what is the purpose of these closings/openings and hope someone could fill me in. Thank you

The reason for openings/closings is for students to change courses if needed. You can have the opportunity to add/drop courses. Its also a great time for people to try getting into a course that was previously full.

As mentioned above, that's about it. It's closed now just to have different registration dates for different levels and programs. It's just a way to have everything organized. Once it re-opens on Monday, it's a chance for everyone to get into courses that were full before. And the last time it closes, it's to generate timetables for everyone and that's when it re-opens in August to be able to switch around core/lab/tutorial sections.
Refer to this: http://registrar.mcmaster.ca /regis...%20FINAL. pdf