You can safely keep your files in the folder during the maintenance work. You will be able to use the workgroup folders again from Monday morning at 8am.
If you are using the \\ru.nl\wrkgrp path to your files, you will reach your files in the same way after the maintenance work.
If you previously shared a link to a workgroup folder or created a shortcut, this link may no longer work. This is because the file location changes due to the work. Changing the link from \\cnas.ru.nl\wrkgrp into \\ru.nl\wrkgrp will allow you to access the folder and reach your files again.