None of you should fast on Friday, unless he fasts a day before it or a day after it
Authentic hadithعن أبي هريرة رضي الله عنه قال: سمعت رسول الله صلى الله عليه وسلم يقول: «لا يَصُومَنَّ أحدكم يوم الجمعة، إلا أن يصومَ يومًا قبله أو بعده».
Abu Hurayrah (may Allah be pleased with him) reported: I heard the Messenger of Allah (may Allah's peace and blessings be upon him) say: "None of you should fast on Friday, unless he fasts a day before it or a day after it."
Explanation
The Prophet (may Allah's peace and blessings be upon him) forbade fasting on Friday, except when being coupled with fasting a day before it or a day after it, or when it is part of a regular fast.
Source · Narrated by Bukhari & Muslim
From the Hadith Encyclopedia (hadeethenc.com).