What we usually do is change the gig status to "Cancelled", then send a chat message out to all the musicians letting them know that this date has been cancelled but the gig will likely be postponed. That way, everyone is in the loop. Make sure you turn off Auto-Pay for the gig, and if you want to keep your financial reports clean, zero-out everyone's rate amounts too.
When you get the new date, just edit the booking's date and change the status back to "Confirmed". Then, post a new chat message asking the musicians if they're available for this new date. Alternatively, you could remove all the musicians now and re-initiate the auto-book process to rebook everyone. It doesn't hurt to mention the old date and that this is the new date, just to avoid any confusion!