Tickets for a whole month go on sale at 14:00 Japan time on the 10th, two months before. A visit on any day in April, for example, goes on sale on 10 February.
Good to know
No tickets are sold at the park itself.
Overseas buyers can get day passes only.
Ghibli Park is near Nagoya, not Tokyo. It is a different place from the Ghibli Museum in Mitaka.