Dua before commencing Fast
وَبِصَوْمِ غَدٍ نَّوَيْتُ مِنْ شَهْرِ رَمَضَانَ
I intend to keep the fast for tomorrow in the month of Ramadan
Dua when opening Fast
ذَهَبَ الظَّمَأُ ، وَابْتَلَّتِ الْعُرُوقُ ، وَثَبَتَ الأَجْرُ إِنْ شَاءَ اللهُ
The thirst has gone, the veins are quenched and reward is certain, if Allah wills