Do not single out the night of Jumu’ah [i.e., Thursday night] from among other nights for praying qiyaam al-layl, and do not single out Friday from among other days for fasting, unless it is a fast that one of you regularly observes.

 
The Prophet (peace and blessings of Allaah be upon him) [Muslim narrated in his Saheeh, al-Siyaam, 1930]