Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
There is no basis for singling out Fridays for visiting graves.
Answering your question, the late Saudi scholar, Sheikh Ibn Baz (may Allah have mercy on him) stated:
There is no basis for singling out Fridays for visiting the graves. What is prescribed is to visit graves at any time of night or day that suits the visitor.
With regard to singling out a particular day or night, this is bidah (innovation) and there is no basis for this.
The Prophet (peace be upon him) said:
“Whoever innovates anything in this matter of ours (Islam) that is not a part of it will have it rejected.” (Agreed upon)
The Prophet also said:
“Whoever does anything that is not a part of this matter of ours (Islam) will have it rejected.” (Muslim)
Almighty Allah knows best.
Source: www.islamqa.info