Artists often approach venues at exactly the moment the calendar is already full, then read the silence as rejection. It usually is not. Venues book in rhythms, and knowing roughly where a room is in its own year tells you a great deal about when to get in touch.
The big dates — the ones a venue builds a season around — are settled a long way out. Anything tied to a holiday, a festival, or a room's own anniversary tends to be handled well in advance, often before the previous one has finished.
Regular programming — the weekly and monthly nights — is booked on a rolling basis, and this is where most artists actually fit. A room filling six weeks ahead is looking six weeks ahead, every week, all year.
Cancellations happen, and rooms need them filled fast. Being the entertainer a booker remembers when they need someone on short notice is worth more than any pitch.
A booker's job is filling a calendar. Anything you do that makes their calendar easier to fill is what makes you easy to book.
The same rhythm is worth being explicit about publicly. If your listing says which nights you programme and how far ahead you work, you get fewer mistimed approaches and better ones — your venue's own page is the place to say it. If your room is not listed yet, add it, or see what a listing includes.
Artists: you can see how rooms near you describe their programming by browsing venues and reading what they publish about their week.