Get First Day of Next and Previous Month

-- To Get First Day of Next Month
SELECT DATEADD(MONTH, DATEDIFF(MONTH, '19000101', GETDATE()) + 1, '19000101')
as [First Day Next Month];
GO

-- To Get First Day of Previous Month
SELECT DATEADD(MONTH, DATEDIFF(MONTH, '19000101', GETDATE()) - 1, '19000101')
as [First Day Previous Month];
GO