How to use grpstats to create statistics using nominal variable from a dataset?

Hello,

Wondering how to use grpstats to create statistics using nominal variable from a dataset?

This is my dataset (example...)

   1  2  3  4  5  6
  Year  Month  Day  Season  AA  BB
 1  2002  1  1  Winter  10  2.5
 2  2002  4  22  Spring  8  5
 3  2002  5  13  Spring  6  3
 4  2002  7  4  Summer  2  2.7
 5  2002  8  25  Summer  1  11
 6  2002  10  16  Fall  13  0.5
 7  2002  11  27  Fall  0.1  4.2
 ...
 1000  2007  2  1  Winter  10  2.5
 1001  2007  4  22  Spring  8  5
 1002  2007  6  13  Spring  6  3
 1003  2007  7  4  Summer  2  2.7
 1004  2007  9  25  Summer  1  11
 1005  2007  11  16  Fall  13  0.5
 1006  2007  12  27  Fall  0.1  4.2
 ...

I would like to calculate statistics as derived from grpstats by using variable 'Month' and/or 'Season'. In this case Months 1,2 and 3= Winter; 4, 5 and 6=Spring; 7, 8 and 9=Summer; and 10, 11 and 12= Fall).

Thank you for your suggestions!