You can take this flight from late August through early December. Travel dates vary, but you'll find the best availability departing Monday, Wednesday, and Sundays. Fares will require a Sunday night stay unless you are leaving on a Sunday.
Searching on Avianca's website, I could find seats departing BOS on Wednesday, October 3, and returning from SJO on Sunday, October 7, 2018.
There is a full list of flight deals today from the Hub so make sure to have a look at our Boston (BOS) departure page.
If you need a place to stay along with this airfare deal, we've got a long list of hotels available for the mentioned dates on Hotelwatchdog.