create function fn_IsWeekDay
(
    @date datetime
)
returns bit
as
begin

    declare @dtfirst int
    declare @dtweek int
    declare @iswkday bit

    set @dtfirst = @@datefirst - 1
    set @dtweek = datepart(weekday, @date) - 1

    if (@dtfirst + @dtweek) % 7 not in (5, 6)
        set @iswkday = 1 --business day
    else
        set @iswkday = 0 --weekend

    return @iswkday
end

EXAMPLE

if dbo.fn_IsWeekDay(@date) = 1
begin
    --do some magic here ;-)
end