ANSWER:

You can fast the day of Arafah alone even if it falls on a Friday or a Saturday, this is an exception just like Ashoora. 

This year 9 dhulhijjah falls on Saturday. Is it permissible to fast on Saturday as there is one hadeeth that forbids us to fast on saturday except if it is a compulsory fasting.

Fasting | August 8, 2019 6:14 am
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If the day of arafah happens to be on a Saturday do we have to fast the Friday as well? I have heard that we are not allowed to fast Saturdays or Sundays by themselves as they are days of the Jews and Christians.