EQUIRRIE. – Two festivals of the most ancient Roman calendar, on 27 February and 14 March, bear this name and are respectively the last and the first of the calendar itself.
The differences between the two celebrations do not emerge from the scanty evidence; they were connected with horse races and took place in the Campus Martius, in honour of Mars. 14 March (one of the only two festivals of the ancient calendar that fall on even-numbered days) is, in fact, defined in some calendars as feriae Marti, while in others it bears the name of Mammarlia.
BULL. W. W. Fowler, The Roman Festivals, 4th ed., Londra 1925, pp. 330–332. Angelo Brelich