There is no way to do this directly in GQL as aggregate functions (SUM, AVG, etc) are not supported in the datastore. Your best bet is to do this manually -- issue a query for the dates that you want (using a projection here for the number could be a good idea), and then average them yourself. Careful though -- if you have a lot of data this will be slow, and eventually will take too long for each request.
It may also make sense for you to keep track of these stats separately. When you added entities you would also have to update the stats. See this page about sharded counters.
Another option if you are doing batch processing is to run this either in a backend or in a map reduce. How up-to-date the numbers you get will depend on how often you run the job, but it will scale. Here is a link to the map reduce documentation.