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.
Kanji doesn't go together much better than this. Both plus day equals both days.
This is a bit of a formal word though, you'll hear important people say it more than your friends.
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.