Why is it customary to visit loved ones' graves before or during the High Holidays?
Traditional Jews make a point of visiting the graves of loved ones during the month of Elul just prior to the onset of the High Holidays, on the day before Rosh HaShanah, or the day before Yom Kippur. Many Jews also visit the cemetery on the loved one’s birthday, an anniversary, or a special personal day. Visitations to the cemetery are not made on Shabbat or Jewish festival holidays.
Jewish tradition discourages excessive mourning and constant cemetery visitations, especially if it becomes an impediment to a return to life. Jeremiah 22:10 proclaims: "Weep ye not [too much] for the dead." Wisely, though, Jewish practice provides for a regular, structured, communal expression of reminiscence, through yahrzeit and Yizkor.
Source: Rabbi Daniel B. Syme, The Jewish Home: A Guide for Jewish Living (URJ Press, 2004)