You want a room that fits everyone and light refreshments that do not break the bank. Three months out is the sweet spot to grab a local hall before someone else snags the date.
A typical US venue rental and snack setup sits right around $800. Splitting that up over a timeline gives you breathing room so you are not scrambling to pay it all in one go.
That intro window is a firm deadline, not free money. If you still carry a balance when the 15 months are up, interest kicks in on what is left.