why does this work

sed -e 's/.*Jun 19//' myfile

And this not

mmm=Jun
dd=19
sed -e 's/.*$mmm $dd//' myfile


both being ran from a shell scrip, not CLI

myfile:
< Jun 19 00:00:00 some data


I've tried quotes round it, no joy, i'm using *. as i'd like to lose the < too.