Hi, Jaynee! The
Night of Joy Christian Music Festival at Walt Disney World does not have a set date, so I can't really tell you to book your trip for a specific time. I can tell you, however, that historically speaking, the event usually takes place either the first or second week in September. This year's festival is being held September 11 and 12.
Things to keep in mind - the festival is very popular, so when the time comes, you'll want to purchase tickets in advance. The performers are announced earlier in the year (although they are subject to change), and they are usually different on each of the two nights, so you might want to check out the line up and see which performers you prefer.
There is a two night ticket. That's definitely something to consider purchasing to really enjoy all of the wonderful music.
I hope this gives you something to think about while you begin your planning!