Different from calculating the last day of a month, it is much easier to calculate the first day of a month.
There are 4 different dates for the last day of a month (e.g., Feb 28, Feb 29, April 30, and March 31), but the first day of a month is always the same.
Please use the formula below to get the first day of a month:
=DATE(YEAR(A2),MONTH(A2),1)
Where A2 is the cell with the date.
YEAR(A2) returns the year of the date, and MONTH(A2) returns the month of the date.