filter data over a period

HI all,

I am trying to filter over a period but I feel I am not getting the answer.

Okay here is my question, I am having 660 (days data) samples where I want to filter 20-100 day period for each season. like 660 days(starting from 06-jun-2011 to 26-mar-2013) is divided into 4 seasons south-west monsoon (2011) 06-jun-2011 to 06-oct-2011 120 samples, similarly North-East monsoon 2011, South-West monsoon 2012, North-East monsoon 2012. every season contains 120 samples. I am attaching the pdf file with image (fig3) 720 data points filtered with 20-100 days period.

http://www.ann-geophys.net/30/1623/2012/angeo-30-1623-2012.pdf

So how to filter it. Thanking you in advance.