We purchased tickets the day before our e-ride night last summer. Last year, reports on the boards were that the E-ride nights were very seldomly selling out (the max is reported to be about 5,000 tickets).
Crowds in Orlando appear to be higher this year and E-ride nights have been suspended for many months, so demand might be higher this go-round. However, I think you'll be fine if you intend to purchase the day before.