grep -o 'ClosedDate="[^"]*"' posts.xml | sed -e 's/ClosedDate="//' -e 's/T.*$//' | sort | uniq -c
This finds questions which have a "ClosedDate" property, extracts the date, and then sorts and counts the number of times each date appears. You can then paste the result into a spreadsheet and have fun. You have to do slightly more to get the days where no questions were closed. Here's a graph of number of questions closed each day, along with a running average:

(This only counts questions that were closed but not deleted. We don't include deleted posts in the public dump.)
]]>Edit: Ah, I see, he was mistaken. That's most certainly not per day.
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@fgdorais: comments don't show the strong initial peak because you need 50 reputation to comment on other peoples' posts. Votes also has a subdued peak because you need to get a small amount of reputation (15) before you can vote.
]]>Any technical reason why comments don't show as strong initial peak in October as the other graphs do?
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All posts, per day

Votes per day

Comments per day

New users per day
