Just for fun, I was looking at packages in April yesterday and it was working just fine. I think they had a notice about April and March being released early.It looks like they release a whole month of packages at the end of each month. Right now you can book packages through Feb 29. On Oct 31st they will release the dates for all of March.
EDIT: Yes, here.
https://reserve.tokyodisneyresort.jp/en/information/detail/?id=2420&lang=&pagetype=VP
https://reserve.tokyodisneyresort.jp/en/information/detail/?id=2426&lang=&pagetype=VP
It stops a few days before golden week.