For an easy snapshot, go to the URL below, which is a Google Flights search for one-way non-stop fares between BOS and SXM on October 1. When it opens, click on the Calendar icon next to "Thu, Oct 1" to get a monthly view, and then use the right-arrow icons to page through the months. Saturday flights begin on 10/10, Tuesday flights begin on 10/27, change to Mon-Wed-Sat for Christmas and New Year's weeks, and then drops to Saturday-only in January. 'Course that could all change tomorrow.

It may be necessary to change your origin city to BOS.

https://www.google.com/flights?hl=en#flt=/m/01cx_.SXM.2020-10-01;c:USD;e:1;s:0;sd:1;t:f;tt:o