TSQL Get Previous values for each group by

Question

Dear Friends,
I have a problem with my TSQL statment. 
I need to get previous value for each day and specific unit. Everything goes fine if I have always 2 unit in each day. But if same day has just one unit or less, I cannot have the previous value in the current record.
The Output is this one:
SK_DAY SK_UNIT
VALUE VALUE_PREVIUS_DAY
20131112 2
30.00 NULL
20131112 3
34.00 NULL
20131113 2
40.00 30.00
20131113 3
45.00 34.00
20131114 2
50.00 40.00
I dont have the second record for 2013-11-14, because in this date I just have value for unit 2.
The final output should include the record:
20131114 3
0 45.00
The Statment I have is:
SELECT MAIN.SK_DAY, MAIN.SK_UNIT, SUM(MAIN.VALUE) AS VALUE
 SELECT SUM(VND1.VALUE) AS VALUE
 FROM FCT_TEST VND1
 WHERE CONVERT(DATE,CONVERT(VARCHAR(10),VND1.SK_DAY))>=DATEADD(dd,-1,CONVERT(DATE,CONVERT(VARCHAR(10),MAIN.SK_DAY)))
AND CONVERT(DATE,CONVERT(VARCHAR(10),VND1.SK_DAY))<CONVERT(DATE,CONVERT(VARCHAR(10),MAIN.SK_DAY))
AND VND1.SK_UNIT=MAIN.SK_UNIT
) AS VALUE_PREVIUS_DAY
FROM FCT_TEST MAIN
GROUP BY SK_DAY, MAIN.SK_UNIT
SQL CREATE SCRIPT:
CREATE TABLE [dbo].[FCT_TEST](
[SK_DAY] [int] NOT NULL,
[SK_UNIT] [int] NOT NULL,
[VALUE] [decimal](18, 2) NULL,
 CONSTRAINT [PK_FCT_TEST] PRIMARY KEY CLUSTERED 
[SK_DAY] ASC,
[SK_UNIT] ASC
)WITH (PAD_INDEX  = OFF, STATISTICS_NORECOMPUTE  = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS  = ON, ALLOW_PAGE_LOCKS  = ON) ON [PRIMARY]
) ON [PRIMARY]
Thank you!!!
PPSQL

Answer 1

Thank you Kalman,
But my problem is not with date! Fortunately I have dates for all days.
The problem is for dimension tables like UNITS that I dont have values for all days, and when I try to get the previous value for previous day and each unit I dont have records. 
20131112
2
 30.00
NULL
20131112
3
 34.00
NULL
20131113
2
 40.00
30.00
20131113
3
 45.00
34.00
20131114
2
 50.00
40.00
20131114
3
 0
45.00 [I NEED THIS RECORD AS IS]
Could you help me??
Thank you!!