The longer version
The cancellation window is the rule behind refunds. Cancel before it closes and the money comes back. Cancel after and the club keeps some or all of it. It is written as hours before the start: 24 hours, 12 hours, 48 hours for a coached lesson.
Clubs set it to balance two things: enough notice to resell the slot, and a rule players find fair enough to book in the first place. A 72-hour window on a weekday court sounds safe but makes players hesitate to book at all.
The window should differ by product. Courts, classes, lessons, and open play have different resale patterns. One rule for everything is the most common mistake, followed by a rule that is not shown at checkout, so the first time a player learns it is when the refund is declined.