"Today is January first."
Kanji has 2 types of readings. Kunyomi, 訓読み (くんよみ) the Japanese reading, and Onyomi 音読み(おんよみ), the Chinese reading.
When children are taught Kanji in Japan, the Kunyomi is written is Hiragana for the native Japanese pronunciation, and the Onyomi is written in Katakana for the borrowed Chinese pronunciation.
Ex: 一 (One)
Japanese Reading: くんよみ
ひと • ひと(つ)
Chinese Reading: おんよみ
イチ • ツイ
Now, a majority of the time when 2 or more Kanjis are placed together like 一日 (ついたち), The Chinese reading (Onyomi) will be pronounced.
Which is why this word is pronounced as Tsuitachi (when counting numbers).
一日 can also be said as いちにち to mean "One day".
This isnt always the case though for Kanjis next to each other to only have the Onyomi reading. Learning Kanji sets as theyre pronounced will take you farther. Rote memorization can work but a lot can be forgotten with the readings. At this stage though I do recommend learning the readings so youre not wondering why its pronounced differently.