Scripting will probably be the easiest way to implement this using ListRasters—Help | ArcGIS Desktop and Cell Statistics—Help | ArcGIS Desktop as Steve Lynch already mentioned before.
To give you a head start with some of the code that will be needed, have a look at the snippet below:
lst_ras = ['r20000125', 'r20000126', 'r20000127', 'r20000128',
'r20000129', 'r20000130', 'r20000131', 'r20000201',
'r20000202', 'r20000203', 'r20000204', 'r20000205']
lst_months = list(set([ras[:7] for ras in lst_ras]))
print(lst_months)
for month in lst_months:
ras_names_month = [ras for ras in lst_ras if ras.startswith(month)]
print("month: {} rasters: {}".format(month, ras_names_month))
You will use ListRasters to get a list of raster names. In my example I created a small list manually (see lines 1 to 3). On line 5 I create a list of the month prefixes and print it on line 6. This will yield:
['r200002', 'r200001']
For each month prefix (line 😎 you can get a list of all the day rasters, which is done on line 9. This list is what you will use with the Cell Statistics as first parameter. The result of line 9 is printed on line 10 as yields:
month: r200002 rasters: ['r20000201', 'r20000202', 'r20000203', 'r20000204', 'r20000205']
month: r200001 rasters: ['r20000125', 'r20000126', 'r20000127', 'r20000128', 'r20000129', 'r20000130', 'r20000131']
