Do you mean evaluating the string as a variable? http://hacktux.com/bash/var...
I should test this out, it would be a handy snippet to have if we can make it work. I'll let you know.
Check this out: you can include shell snippets in other shell snippets. So you could (and this is untested) take your reusable function and make a snippet out of it. Call it ",datemath" or something. Edit the TARGET= line to:
TARGET=${!day}
Then, in the "Next Tuesday" snippet, you can just do:
#!/bin/bash
day=TUE
%snippet:,datemath%
That would save repeating the script over and over for other day snippets...