You want it installed before whatever birds are likely to nest in it start looking for nesting sites. When that is depends on what kinds of birds are common in your area, and how big the opening is. I would think the earliest possible date any birds would be nesting in Southeastern Mass would be March. If you are trying to attract a specific kind of bird you might not want to put it in until a short time before that bird nests and make sure the opening size is right. If you don't care what bird uses it, have it in as soon as possible.

Key things with bird houses are:
a.) You want to be able to empty it out and clean it in the winter, and;
b.) If you are trying to attract small songbirds, surround opening with a metal washer do so crows, bluejays and squirrels can't widen the opening to get in.
c.) You want it it hard for squirrels and cats to get to...at the top of a smooth pole far from any spot a squirrel could leap from, hanging from longish chains, etc.

Here is a link that might be useful: When to Put Up Bird Houses