I have a year's worth of inventory data, and I want to extract the first and last day of each month's data. What is the best way to do this? Thanks
That is a pretty wide-open description and could use more details if there are complications in the data you are using. But here is a basic idea.

First day of month is easy:  YYYYMM01

Last day of month can easily be calculated by:

   DECLARE @FirstDayOfMonth DATETIME; -- Use DATETIME for older SQL Server versions
   SET @FirstDayOfMonth = '20150601' -- Safest format for dates.
   SELECT DATEADD(DAY,-1,(DATEADD(MONTH,1,@FirstDayOfMonth)));


This trivializes finding the last day of the month.  It is one day before the first day of the next month. No need to calculate leap years, lengths of  months, et cetera.