The calendar of a parish pilgrimage runs backward from the departure. International flights and hotel blocks are best held about a year out. Pilgrims need time to decide, to talk with a spouse, to arrange leave and to pay in stages rather than all at once. Add those together and the parish that begins fifteen months ahead is comfortable, while the one that begins six months ahead is choosing from what is left.
The shrines themselves set no single deadline, which is a reason to go early rather than late. The Shrine of Fátima asks groups to register in writing "in time" with the name of their priest, the dates and the number of pilgrims, and warns that private Masses and chapels are hard to obtain on the 12th and 13th of each month from May to October. St. Peter's Basilica asks visiting priests to write ahead to its celebrations office. The Auschwitz Memorial, a stop on most Poland routes, asks that guides be reserved at least two months before the visit.
Good to know before you go: whoever starts the planning, a parish secretary, a lay leader or the pastor himself, the first thing to settle is the priest. His bishop or religious superior may need to approve an absence of that length, and nothing should be announced under his name until he is certain he can go. His own share of the timeline is light: he approves the plan and the announcement, then spends the months before departure preparing his people spiritually.