Weekly Aggregation based on everyday date

Hi All,

i have a table with 3 columns. Id , MeasureDate,Measure. Here   Id +MeasureDate as primary key.  In this table i want add a column Avg.  if i am running today 2/12/2014,2/5/2014,01/29/2014 and so on avarage value will be replaced in the Avg column .

Give some ideas.

Thiru