Here are three T-SQL functions that you can use to extract the month from a date in SQL Server.

MONTH()

The most obvious function to use is the MONTH() function. This function accepts one argument: the date.

DECLARE @date date = '2020-10-25';
SELECT MONTH(@date);

Result:

10

DATEPART()

The DATEPART() function accepts two arguments: the first argument is the part of the date that you want to extract, and the second argument is the actual date that you want to extract it from.

DECLARE @date date = '2020-10-25';
SELECT DATEPART(month, @date);

Result:

10

In this example I used month as the first argument. You also have the option of using mm or m. Whichever one you use, the result is the same.

DECLARE @date date = '2020-10-25'
SELECT 
  DATEPART(month, @date) AS month,
  DATEPART(mm, @date) AS mm,
  DATEPART(m, @date) AS m;

Result:

 +---------+------+-----+
 | month   | mm   | m   |
 |---------+------+-----|
 | 10      | 10   | 10  |
 +---------+------+-----+ 

FORMAT()

The FORMAT() function can also be used to return the month.

DECLARE @date date = '2020-10-25';
SELECT FORMAT(@date, 'MM');

Result:

10

You can alternatively use MMMM to return the full month name, or MMM to return the short month name.