Looking for suggestion - Quantifying data

I have a chart that lists the number of "observations" completed each day throughout the month. Most months, there are very few observations early in the month, and as the month goes on, more observations are completed. This causes an upward slant on my chart.

Essentially, my goal is to have observations completed evenly throughout the month.

What I'd like to have is a way to measure the progress being made each month. I don't think I'm looking for just an average though. Maybe I am, it just seems too easy.
Perhaps I need to get the percentage each day represents and then weigh it somehow? Any thoughts?

Book1.xls

I marked it as solved, but then began thinking further. I'm still not sure I know the answer. Thanks for your help