It is not permissible to single out a certain day of the year, like Friday or the first day of Rajab or any other day, for visiting graves. There is no evidence to support this. It is, however, permissible to visit graves on any day possible, without specifying a certain day. (Part No. 9; Page No. 114) Permissibility to visit graves is based on the Hadith in which the Prophet (peace be upon him) said:
Visit the graves, for they remind you of the hereafter.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.