The statements are repeatedly performed until the condition is met.

ALTER PROCEDURE [SchoolDemo].[UpdateSlot]
    @EveningID int,
    @SlotNotes varchar(250)


AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;


declare @StartDateTime varchar(50)
declare @EndDateTime varchar(50)

declare @SlotInterval int

select @StartDateTime= (select StartDateTime from dbo.tblEvening where EveningID=@EveningID)
select @EndDateTime= (select EndDateTime from dbo.tblEvening where EveningID=@EveningID)
select @SlotInterval= (select SlotInterval from dbo.tblEvening where EveningID=@EveningID)


while DateDiff(minute,@StartDateTime, @EndDateTime) > 0
    begin

        insert into dbo.tblSlot (EveningID,SlotStartTime,SlotEndTime,SlotEnabled,SlotNotes)
        values(@EveningID,@StartDateTime,dateadd(minute,@SlotInterval,@StartDateTime),1,@SlotNotes)
        select @StartDateTime=(dateadd(minute,@SlotInterval,@StartDateTime))

    end