" wrote:
A B C
date value
14/01/11 400,00
16/01/11 402,00
[....]
11/02/11 428,00
13/02/11 430,00

I need a formula to sum in column C cells those match
criteria January or February or other month oy the year.

You could write:

=SUMPRODUCT(--(MONTH(A1:A100)=1),B1:B100)

The double-negative (--) converts TRUE and FALSE to 1 and 0, which
SUMPRODUCT requires. The month number 1 represents January.

Alternatively, if C1 contains a date like 01/01/11 (Jan 1 2011), which you
can format as Custom mmmm if you want to see January, you could write:

=SUMPRODUCT(--(MONTH(A1:A100)=MONTH(C1)),B1:B100)