In SQL Server you can use DAY function to get the day (an integer number) of the specified datetime. In PostgreSQL you have to use EXTRACT(DAY FROM datetime) function.
SQL Server:
-- Get the day SELECT DAY('2022-12-28'); # 28
PostgreSQL:
-- Get the day SELECT EXTRACT(DAY FROM DATE '2022-12-28'); # 28
For more information, see SQL Server to PostgreSQL Migration.