Same Day
noun
The instant day is the day that's going on this instant. It's the same day as now.
The reading for 即 is the normal one. For 日 you're going to want to use the じつ reading, because, the truth (じつ) is that it's the same day today.
この回転肉焼き器は、注文日の即日に発送致します。
We send out this rotisserie on the same day as you order.