declare
    @dt         datetime,
    @StartDate  datetime,
    @EndDate    datetime

create table #Dates (dt datetime)
set @dt = @StartDate
while @dt<=@EndDate
begin
    insert #Dates(dt)
    values (@dt)
    set @dt = @dt +  1 
end

select D.dt
    from #Dates D
    where not exists (select top  1  'X' from [Заказы] where [дата заказа] = D.dt)

drop table #Dates

declare
    @StartDate  datetime,
    @EndDate    datetime

select D.Dt
    from (select dateadd(dd,(select count(*) from sysobjects o2 where o2.id<o1.id),@StartDate) as Dt
                from sysobjects o1) D
    where
        D.Dt between @StartDate and @EndDate and
        not exists (select top  1  'X' from [Заказы] where [дата заказа] = D.dt)