The first meaning is one day, and it has the same reading as the kanji you learned, so you should already know this reading.
The reading for first day, however, is a big old exception. It doesn’t take the on’yomi or kun’yomi readings, and has its own special (weird) readings. Here’s a mnemonic to help you remember this one:
On the first day of your new relationship you can get two touches (ついたち) only. That’s the rule! You can touch their hand and maybe high five. But that’s all you get. Just think when you’re on your first day of dating, two touches! Two touches only!
I was studying for the test all day yesterday.
The fact that an American and French couple succeeded to climb Mt. Everest on June 1st is still fresh in people's memory.
Did you really read this book in one day?