What is the ruling on singling out Friday for visiting graves?

There is no basis for singling out a specific day or night for visiting graves. The prescribed way is to visit them at any time it is easy, because singling out a specific time is an innovation (bid‘ah), based on the Prophet's (peace and blessings be upon him) saying: "Whoever introduces into this matter of ours that which is not from it, it will be rejected," and his saying: "Whoever does a deed that is not in accordance with our matter, it will be rejected."