FROM CalenderTable LEFT OUTER JOIN TableWithOtherDatesInIt
That's the starting point. Your where clause was filtering out any nulls and essentially turning the join into an inner join.
You don't want to be doing any messy date to string conversions, they're both date columns, so just compare directly. CAST as DATE if one contains times.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability