alter table TableA
add DateColumnYear as datepart(year, datecolumn),
DateColumnMonth as datepart(month, datecolumn),
DateColumnDay as datepart(day, datecolumn)
go
create index IDX_TableA_DateParts on dbo.TableA (DateColumnDay, DateColumnMonth, DateColumnYear)

select ...column list...
from dbo.TableA
inner join dbo.TableB
on TableA.DateColumnYear = TableB.DateColumnYear
and TableA.DateColumnMonth = TableB.DateColumnMonth
and TableA.DateColumnDay = TableB.DateColumnB

alter dbo.TableA
add DateColumnDate as cast(convert(varchar(25), DateColumn, 102) as datetime)
go
create index IDX_TableA_DateColumnDate on dbo.TableA (DateColumnDate)

SELECT...
FROM....
WHERE
CASE WHEN @MyParam IS NULL THEN 1
   WHEN @MyParam = table1.Column1 THEN 1
   ELSE 0
END = 1