The World Federation One Stop Fiqh

Ruling 2674

If a person vows to fast on a specific day every week, such as Friday, in the event that Eid al-Fiṭr or Eid al-Aḍḥā6 falls on a Friday, or if on Friday the person has another legitimate excuse to not fast, such as travelling or ḥayḍ, he/she must not fast on that day but must keep a qaḍāʾ fast. 

6 Eid al-Fiṭr is on the 1st of Shawwāl and Eid al-Aḍḥā is on the 10th of Dhū al-Ḥijjah. It is unlawful to fast on these days. See Ruling 1707.