This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. Note that 日 uses the reading じつ for this one instead of にち.
I’ll be in a meeting both days.
The tofu festival had good weather for both days.
I attended the iPad lecture session on both days, but I still have no clue how to use it.