You have a n day streak!
Bash, 102 bytes

find ! -newerat 0-1day -delete
touch -a a
echo You have a $((1+(`stat -c %X-%Y a`)/86400)) day streak!


Warning, do not run in any folder you care about. It deletes any file not accessed in the last day in the working directory.

Uses a file a to store data, using the accessed/modified timestamps.