I DO NOT have a complete solution. But here is the guts that will give you the day of the week for the file name given the date:
#!/usr/bin/env bash
file="2015-02-09 Some file.txt"
when="${file%%\ *}"
day=$(date -j -f "%Y-%m-%d" "${when}" +%A)
new_name="${when} $day ${file#*\ }"
echo mv "${file}" "${new_name}" # remove the echo when the code is debugged
You could craft something in Automator using the "Run Shell Script" and create a drag and drop app that runs a shell code using parts of the above (you would have to fetch the file(s) to rename from the Automator input, and implement looping if more than 1 file. Actually not too difficult.
In Automator. Select New Application.
Drag "Run Shell Script" to the right work space window
Change "Pass Input" to "As Arguments"
Change the template from
for f in "$@"
do
echo "$f"
done
to something like:
for f in "$@"
do
file="${f}"
when="${file%%\ *}"
day=$(date -j -f "%Y-%m-%d" "${when}" +%A)
new_name="${when} $day ${file#*\ }"
echo mv "${file}" "${new_name}" # remove the echo when the code is debugged
done
And remove the 'echo' from in front of the 'mv' when you are happy with the code.
NOTE: if the file format is NOT exactly "YYYY-MM-DD<SPACE>other.stuff", the script will fail, and it may fail in a bad way. so do be careful with automation.