Which is the most correct and accurate calendar for prayer times in Toronto, Canada, especially when there is a 20-minute difference between the Umm Al-Qura calendar and the ISNA calendar, and is it obligatory to re-perform the Fajr prayers that were prayed based on the Umm Al-Qura calendar?

No calendar can be definitively declared accurate. The fundamental principle for determining the entry of prayer times is based on the Shar’i (legal) signs. Calendars are used for approximation of times, but they should not be relied upon unless they have been approved by trustworthy scholars. If there is doubt about the entry of a prayer time due to conflicting calendars, one must wait until certainty or strong probability (ghalabat al-dhann) of its entry is achieved, because prayer should not be performed with doubt concerning the entry of its time. If certainty or strong probability is achieved by the later time, then one must act according to it.