There’s nothing wrong with telling other people you’re fasting as long as you’re not doing it to score ‘spiritual points’. The Bible is clear in Matthew 6 that it shouldn’t be obvious to people around us that we’re fasting because we’re looking depressed, or tired, or unhappy. Instead Jesus tells us to continue to take care in our appearance and our attitude while we’re fasting. The general rule is don’t go around telling everyone that you’re fasting – because you want the reward that God has for you by keeping it between you and him. But if someone asks why you’re not eating – there’s nothing wrong with saying ‘Oh I’m fasting today’.