Converting weekly data with dates to montly averages

Hi there.
I have a problem that I have not found a solution to yet.
I have plenty (10000) of data divided into to two columns A and B.
A contains the weekly dates and B contains the data.
I would like to convert B to another column D that contains monthly averages of B.
And a column C with year and month info.
Right now I do it manually and it takes forever.

Thanks in advance

We will need an example to properly help you out. Have you looked into the possibility of including column C and D to retrieve the month and year from column A, then insert a Pivot Table or use SUMIF to obtain your averages/ sums?

Here comes an image of what I have.
I want to take average of the weekly data to make a monthly column D, with the month on Column C
Example of weekly data.JPG