Create table #x1
(
iSchoolCode int,
iSchoolYearCode int,
dtCalendarDate datetime,
iCalendarDayId int
)

drop table #x1

insert into #x1 values (301,2012,'2012-08-10 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-11 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-12 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-13 00:00:00.000',4259)
insert into #x1 values (301,2012,'2012-08-14 00:00:00.000',4260)
insert into #x1 values (301,2012,'2012-08-15 00:00:00.000',4261)
insert into #x1 values (301,2012,'2012-08-16 00:00:00.000',4262)
insert into #x1 values (301,2012,'2012-08-17 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-18 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-19 00:00:00.000',4263)
insert into #x1 values (301,2012,'2012-08-20 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-21 00:00:00.000',4264)
insert into #x1 values (301,2012,'2012-08-22 00:00:00.000',4265)
insert into #x1 values (301,2012,'2012-08-23 00:00:00.000',4266)

iSchoolCode	iSchoolYearCode	dtCalendarDate			iCalendarDayId	ROW_Number	
301			2012	2012-08-10 00:00:00.000		NULL		NULL					
301			2012	2012-08-11 00:00:00.000		NULL		NULL			
301			2012	2012-08-12 00:00:00.000		NULL		NULL			
301			2012	2012-08-13 00:00:00.000		4259		1
301			2012	2012-08-14 00:00:00.000		4260		2
301			2012	2012-08-15 00:00:00.000		4261		3
301			2012	2012-08-16 00:00:00.000		4262		4
301			2012	2012-08-17 00:00:00.000		NULL		NULL			
301			2012	2012-08-18 00:00:00.000		NULL		NULL			
301			2012	2012-08-19 00:00:00.000		4263		5
301			2012	2012-08-20 00:00:00.000		NULL		NULL			
301			2012	2012-08-21 00:00:00.000		4264		6
301			2012	2012-08-22 00:00:00.000		4265		7
301			2012	2012-08-23 00:00:00.000		4266		8

SELECT   iSchoolCode ,
		iSchoolYearCode,
        dtCalendarDate ,
        ROW_NUMBER() OVER ( ORDER BY dtCalendarDate ) RowNum
FROM    #x1
WHERE iCalendarDayId IS NOT NULL
UNION ALL
SELECT   iSchoolCode ,
		iSchoolYearCode,
        dtCalendarDate ,
        NULL RowNum
FROM    #x1
WHERE iCalendarDayId  IS  NULL
order by dtCalendarDate

Create table #x1
(
iSchoolCode int,
iSchoolYearCode int,
dtCalendarDate datetime,
iCalendarDayId int
)



insert into #x1 values (301,2012,'2012-08-10 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-11 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-12 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-13 00:00:00.000',4259)
insert into #x1 values (301,2012,'2012-08-14 00:00:00.000',4260)
insert into #x1 values (301,2012,'2012-08-15 00:00:00.000',4261)
insert into #x1 values (301,2012,'2012-08-16 00:00:00.000',4262)
insert into #x1 values (301,2012,'2012-08-17 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-18 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-19 00:00:00.000',4263)
insert into #x1 values (301,2012,'2012-08-20 00:00:00.000',NULL)
insert into #x1 values (301,2012,'2012-08-21 00:00:00.000',4264)
insert into #x1 values (301,2012,'2012-08-22 00:00:00.000',4265)
insert into #x1 values (301,2012,'2012-08-23 00:00:00.000',4266)


SELECT *

FROM

#x1 X1

LEFT JOIN
(
SELECT  
iCalendarDayId
,ROW_NUMBER() OVER ( ORDER BY dtCalendarDate ) RowNum
FROM    #x1
WHERE
iCalendarDayId
IS NOT NULL
) AS Row
ON x1.iCalendarDayId = Row.iCalendarDayId

drop table #x1