first day of the month; the first
Refers specifically to the first calendar day of a month. Not used for 'one day' duration; that is いちにち.
来月の一日は月曜日です。
The first of next month is a Monday.
４月１日はエイプリルフールです。
April 1st is April Fools' Day.
The new school term starts on April 1st.
The new rates will apply starting from the first of next month.
十日 (とおか) and 一日 (ついたち) are both irregular native-Japanese readings used for dates, and beginners often confuse which day each one refers to.
Rarely used kanji form; mostly seen in formal or traditional contexts.