Tickets for a whole month go on sale at 10:00 Japan time on the 10th of the month before. A visit on any day in April, for example, goes on sale on 10 March.
Good to know
Tickets are sold through Lawson Ticket. Create the account before sale day so you are not filling in forms at 10:00.
Every ticket is for a set date and entry time. There are no tickets at the door.
Weekends and school holidays go first. Have a second date ready.