This is an Orthodox feast held each year on December 26. A synaxis is a feast day in honor of a saint with special services (matins, vespers, etc.) written for that day. It typically occurs following a major feast day and recognizes a saint that participated in the event of that feast. So, for instance, the Synaxis of St. John the Forerunner falls on January 7, the day after Theophany. The Synaxis of the Theotokos, fittingly, occurs the day after the Nativity of Christ. This feast is likely the oldest feast of Mary within the Church and the beginning of her formal veneration in the liturgical cycle of the Church.
The Akathist to the Holy Virgin Theotokos by St. Romanos the Melodist is normally done each Friday during Great Lent, but this post on the Synaxis seems like an appropriate time to share it.
Frederica Mathewes-Green reads her own translation of the hymn in this podcast.