To get last date of month uses sql server date function and EOMONTH function.
Syntax
EOMONTH ( start_date [, month_to_add ] )
Examples
SELECT EOMONTH (GETDATE());
SELECT EOMONTH (SYSDATETIME());
SELECT EOMONTH (CURRENT_TIMESTAMP);
SELECT EOMONTH (SYSDATETIMEOFFSET());