Open the query builder - bring in your table. Press the Epsilon (Totals) symbol in the query builder.
Assuming your Collect date field only has dates and not date/time elements bring that into the fields to output, and select group by in the totals line.
Ah - well you didn't tell us about lots of other fields did you
You may want to have a read about sub queries http://allenbrowne.com/subquery-01.html - but this is not really beginner level, so perhaps you should simply do the average as a first saved query then add that back to your other query fields with a suitable join.