Mark the first commit from each day as "reword" and the rest as "fixup":
reword xxxxxx First commit from 2013-04-01
squash xxxxxx another commit from 2013-04-01
squash xxxxxx another commit from 2013-04-01
reword xxxxxx First commit from 2013-04-02
etc
This will prompt you to write a new commit message for each day.
Note that doing commits on a time basis is almost always the wrong thing to do. For tools like git bisect
to work correctly, each commit in a repository should represent a "bite-sized" change.