Which table should a foreign key live on
As a rule of thumb, you should add a foreign key on the child table referencing the parent table. In your case it appears that WorkingHoursDetail is the child table and WorkingHours the parent table. 

You can identify the parent table by asking which table can exists on its own without the presence of the other table. In your case WorkingHours can exists without WorkingHoursDetails (but not the other way around, so WorkingHours appears to be the parent table. 

Alternatively you can look at the data flow when inserting new records. If both tables are empty, you should have to insert a row in WorkingHours first and then a record in WorkingHoursDetail. This makes WorkingHours the parent table and therefore the foreign key should be added on WorkingHoursDetail referencing WorkingHours.