Regardless of time of year, you should make at least some ressies in advance, particularly for any character meals and any restaurants that are your favorites and you definitely want them (you will usually find that your favorites are also the favorites of huge numbers of others and they may fill for dinner times before you get there). Otherwise, you will likely find that you can make ressies at others even while you are there, although you should do so at least a day before going to dinner at a restaurant.