A single yellow line simple means that parking is not permitted at certain times - which should be displayed on a plate.
In some seaside places, for example, there is no parking between May and September. In some market towns there may be no parking on Wednesdays and Saturdays - market days.
It could be that Sundays are 'no parking' in that particular area. Always check the plate.
Incidentally, the plates hardly ever state when parking is allowed. Usually it specifies when you cannot park.