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 a new relationship you just tweet a cheesy (ついたち) line at each other. You're so in love you can't speak normally, so you just use bird calls and cheesy lines to communicate.