Hi Felipe,
So the first step would be to get the calculated metric

Once done you can get the values in two ways in Analysis Workspace:
1. Months broken down by Days

Here the months will show the average visits across all months (example: 90388). However, the day breakdown will show the average daily visits across each month (example: 3026, 3052, 2975 and 3004). I have collapsed all days to have just the average daily number shown
2. Cross-tab between months and days

I have got the days as a dimension column in the table. The months are under the average visits metric as column each. Again, I have the day metric collapsed, so that only the concerned number is seen.
I understand, both these methods are not very refined, but they are easy to create. If want to have refined and a solution that you do not have to tweak continuously, I would recommend Report Builder.