I've been doing occasional weekend day trip to Boston and have been parking at Alewife without any issues. From what I've read on weekdays the parking garages fill up very early in the morning.

What time of day do the garages start to have free spaces again? If it matters I'm open to parking at any of the outer terminus stations: Oak Grove, Alewife, Riverside, Braintree, whatever. It doesn't need to be Alewife.

There's a weekday concert in mid September I want to see. If I knew I could reliably park sometime in the afternoon (or late morning), that'd give me the push to go ahead with getting a ticket.