A: It is dislikeable to make a vow to do something, as it was authentically reported that the Prophet (may peace be upon him) forbade making vows, saying, It (a vow) does not prevent anything (that has to take place). In another narration: It (vow) does not bring good, but it is only from the miserly that something is extracted through it. (Related by the Six Hadith Compilers (Al-Bukhari, Muslim, Abu Dawud, Al-Tirmidhy, Al-Nasa'y, and Ibn Majah) except Al-Tirmidhy from the Hadith reported by Ibn `Umar (may Allah pleased with them))

However, if you made a vow of obedience such as the vow mentioned above, you have to fulfill it, as it was authentically reported that the Prophet (peace be upon him) said, Whoever vows that he will obey Allah should obey Him; and whoever vows to disobey Allah should not disobey Him. (Related by the Six Hadith Compilers (Al-Bukhari, Muslim, Abu Dawud, Al-Tirmidhy, Al-Nasa'y, and Ibn Majah) except Muslim from the Hadith reported by `Aishah (may Allah be pleased with her).

The vow you made is a vow of obedience which you have to fulfill even at different times if you did not intend such Sawm to be successive. You have the option to delay such Sawm until you can observe it.

May Allah grant us success! May peace and blessings be upon our Prophet Muhammad, his family and Companions!