Depends on the length of your resort reservation and if you are staying at a Disney owned resort or not.
If you are staying at a Disney owned resort you will need to go through My Disney Experience and you can make reservations for your check in day through check out day up to a maximum of 10 days (180 + the length of your stay up to 10 days.) If you are staying longer than 10 days on the same reservation your 11th day will open up the day after tomorrow, the 12th on the day after that.
If you are doing a split stay tomorrow will be the 180 day for your first resort reservation and then you will have to wait until the 180 day for the second resort reservation to make dining reservations for that stay.
If you are staying at a non-Disney owned resort or offsite it will be one day at a time, 180 days before the day you want to dine at the restaurant.