When I entered some date, I want auto week formulation in another cell.
Hope I made my question clear.

Date Month Week
6-Nov-09 Nov 1st Week

Not really. This could mean a lot of different things.

Is the first week of a month the first 7 days, regardless of where it lands in the week?

For instance, Dec 5, 2009 is a Saturday. Is that:

1) the first week of December because it's one of the first 7 days of the month?
2) the last week of November because the beginning of this week was still in November?

Is Dec 7 2009 considered:
1) 1st week of Dec (because of 7 days)
2) 1st week of Dec (because the first real week started on Dec 6)
3) 2nd week of Dec?

This stuff is anything but obvious.