Apparently we are never ON an island in Gaelic, always IN an island ;)
But that's not the point. This is a translation into English, in which in many dialects we are ON an island, so that translation should also be accepted.
I would use di dòmhnaich
Both are taught on the course, separately. If you are asked to translate 'Sunday' from English to Gaelic, both 'Didòmhnaich' and 'Latha na Sàbaid' will be accepted though.
The day after the Sabbath? Latha na Sàbaid