I say, and with God's help: This is a voluntary fast that is recommended on Mondays, and it can be performed on Sunday as well, and God willing, it will not diminish your reward. As long as you intended it from the night and dawn broke with this intention, fasting has become obligatory for you, so it is not permissible for you to break your fast. If you do break it, you must make up for it. And God knows best.