You are never ready, and you do not need to be
The instinct is to hold invitations back until every square is filled โ venue confirmed, cake chosen, playlist accused of nothing. So the invites go out late, the replies arrive panicked, and the caterer gets a headcount on Wednesday for Saturday. Waiting for completeness is itself a decision, and usually the expensive one.
A party is usually two facts deep long before it is complete: a date, a rough place, someone's name worth celebrating. A planner that waits for the finished version wastes the useful period; one that starts with the two facts and marks the rest honestly puts that period to work.