I've got offered a job which you work on a Saturday aswell as weekdays I need an excuse why i cant do saturday?

If you do not want to work Saturdays tell them you are not interested. There is no excuse you can offer that they will find acceptable. They are trying to fill a position with Saturday coverage. If you will not provide Saturday coverage turn down the job. They are not looking for someone just to work weekdays and they will not work around your schedule.