Вот вариант с использованием GNU datamash для усреднения:
datamash -t, groupby 4 mean 1 < file |
awk -F'[,:]' '{printf "Average for %dth %s is %f\n", $1,$2,$4}'
Average for 25th May is 2940.020000
Average for 26th May is 1410.020000
Average for 27th May is 2677.570000