you do like this
CREATe Table #Temp (Number NVARCHAR(10),InvDate DATE,Amount DECIMAL(18,3))
insert into #Temp VALUES ('1001','2015-01-01',100)
insert into #Temp VALUES ('1002','2015-01-03',200)
insert into #Temp VALUES ('1003','2015-01-05',300)
insert into #Temp VALUES ('1004','2015-01-10',400)
insert into #Temp VALUES ('1005','2015-01-12',500)
insert into #Temp VALUES ('1006','2015-01-18',600)
insert into #Temp VALUES ('1007','2015-01-20',700)
insert into #Temp VALUES ('1008','2015-01-21',800)
insert into #Temp VALUES ('1009','2015-01-27',900)
insert into #Temp VALUES ('1011','2015-02-01',1100)
insert into #Temp VALUES ('1012','2015-02-03',1000)
insert into #Temp VALUES ('1013','2015-02-05',1300)
insert into #Temp VALUES ('1014','2015-02-10',1400)
insert into #Temp VALUES ('1015','2015-02-12',1500)
insert into #Temp VALUES ('1016','2015-02-18',1600)
insert into #Temp VALUES ('1017','2015-02-20',1700)
insert into #Temp VALUES ('1018','2015-02-21',1800)
insert into #Temp VALUES ('1019','2015-02-27',1900)
insert into #Temp VALUES ('1021','2015-03-01',2100)
insert into #Temp VALUES ('1022','2015-03-03',2200)
insert into #Temp VALUES ('1023','2015-03-05',2300)
insert into #Temp VALUES ('1024','2015-03-10',2400)
insert into #Temp VALUES ('1025','2015-03-12',2500)
insert into #Temp VALUES ('1026','2015-03-18',2600)
insert into #Temp VALUES ('1027','2015-03-20',2700)
insert into #Temp VALUES ('1028','2015-03-21',2800)
insert into #Temp VALUES ('1029','2015-03-27',2900)
DECLARE @Start int = 3
DECLARE @End int = 12
SELECT *
FROM #temp
WHERE InvDate>= DATEADD(mm,DATEDIFF(mm,0,InvDate),@Start -1)
AND InvDate < DATEADD(mm,DATEDIFF(mm,0,InvDate),@End)