How to set a date to the first / last day in TSQL

To set a date to the first / last day in TSQL:

First day:

SELECT DATEADD(mm, DATEDIFF(mm, 0, GETDATE()) + 1, 0)

Last Day :

SELECT DATEADD(dd, -1, DATEADD(mm, DATEDIFF(mm, 0, GETDATE()) + 1, 0))