MIT has several distinct systems for reserving rooms. There's a whole bunch listed in the Event Planning Guide (most listed on
http://web.mit.edu/eventguide/other/independent.html; the big systems have their own pages linked from the sidebar or top bar on that page).

The two main groups of rooms (which are almost always what I've used, although Tech Squares has occasionally used some others) are:

• CAC spaces --- large event spaces (including La Sala). Those are reserved through CAC EMS (https://cac-ems2.mit.edu/EmsWebApp/) (as are DAPER (athletic) spaces). Generally speaking, CAC EMS will tell you actual availability and if you reserve the room online you have it. You need to know the username and password to use that system, but you do *not* need to be on the reservation signatories list in Engage. (However, the primary contact for any scheduled events should be listed in Engage.)
  • Booking for CAC spaces opens in November for the following academic year (e.g. November 2019 for academic year 2020-2021). Booking for summer is separate from academic year booking and opens the spring before the summer (e.g. spring 2020 for summer 2020). The timing is announced in advance to squares@mit.edu. 
  • We generally try to reserve rooms as quickly as possible, immediately after the books open at 9 AM.
  • Dates and other details about Opening of the Books can be found at http://studentlife.mit.edu/cac/resources-forms. 
• Schedules spaces --- most classrooms. Those are reserved through http://web.mit.edu/registrar/classrooms/reserve/index.html. You can't see what rooms are available on that site, There's no username or password for that system; instead, Schedules will actually check Engage for authorization (at least, in theory, and I've seen a couple emails that suggest that they do in practice too).

Rooms We Reserve has some candidate rooms, mostly larger rooms suitable for Tuesdays, but the "Classrooms" section has a small list of smaller ones. The Workshop rooms page focuses more on smaller rooms. Room preferences by event type has a rough ordering of preferred rooms for common Tech Squares event types. Common room setups lists common setup instructions for different rooms used for Tech Squares events.