Explanation of the hadith
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.
None of you should fast on Friday, unless he fasts a day before it or a day after it
Authentic hadithNarrated by Bukhari & MuslimHadeethEnc #4449
عن 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».
English translation
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."
Voluntary Fasting