|Use a pivot table.|
Guessing you're on Excel 2003 (most people are...).
Data / Pivot Table. Put dates in the rows, and values in the main "data items" part.
Right click anywhere on a date and choose Group... (in the Group and Show Detail submenu). "Months" should be an option.
Then right click anywhere in the main data. In field settings, summarize by Average.
Should work fine. Excel 2007 is very similar, but PivotTable is in the "Insert" ribbon rather than the "Data" one.