Most people who want a Princess meal eventually get a reservation, but it's possible it won't be on your 180th day.
You will be able to call or go online on your 180th day, but there is an advantage for people who are staying onsite. They are able to make reservations for 180 + 10 meaning on their 180th day, they can book meals for the first ten days of their vacation. For example, if their check in day is Sept 1 they will be able to make reservations through Sept 10 all at once. This means that it is sometimes harder for offsite people to get their reservations even on their 180th day, especially for very popular restaurants.
If you aren't able to get your first choice right away, it's disappointing, but keep checking online because people change their minds all the time and openings come up regularly, especially for the princess meals because it requires a credit card guarantee. People will cancel if they change their minds so that their credit card isn't charged.
Also online booking starts at 6 AM Orlando time and calling isn't until 7 AM so you would get a little more headstart if you try online.