Currently the reservation system is being updated. They are accepting reservations up through February 22.
Beginning on or about November 21, they will open the reservation window back to 180 days. So on November 21 those arriving from February 22 - May 20 will be able to make their reservations.
There are three exceptions. Cinderella's Royal Table, Chef Mickey's, and Victoria & Albert's. Those reservations can be made at 180 days throughout computer update the process. (From reports of other Dis'ers, the 180 +10 also currently applies for onsite guests for these three restaurants)