averages within columns

[cac13]

I have a long series of data set up in the following manner:

July 2 a
July 3 a
July 4 a
July 5 b
July 6 b
July 7 c
July 8 a
July 9 a
July 10 a
July 11 a
July 12 b
July 13 b
July 14 b