The answer to this question can be found in Chapter 21 of this novel, which describes Election Day, which is the annual installation of magistrates. In this ceremony, the narrator tells us, something of the majesty and importance of ceremonies in old England is captured and there is much pomp and circumstance:
The fathers and founders of the commonwealth--the statesman, the priest, and the soldier--deemed it a duty then to assume the outward state and majesty, which, in accordance with anitque style, was looked upon as the proper garb of public or social eminence. All came forth, to move in procession befoe the people's eye...
In addition we are told that the people are allowed to relax their usual Puritan habits and customs, and to enjoy themselves more than would be normally publicly sanctioned. The Election Day is therefore an important festival in the Puritan calendar, and of course is particularly important in this story as Dimmesdale plans to preach his last sermon here before leaving with Hester to start a new life.