It has become customary for mourners to gather at the cemetery for a ceremonial "unveiling" of the headstone of a loved one who has died. This is a powerful ritual moment to bring together friends and relatives to reflect on the legacy of the deceased and to pay honor to their memory.
In general, the custom is for the unveiling to happen at approximately the one year mark following a death (see Lamm, "The Jewish Way in Death and Mourning," Klein, "A Guide to Jewish Practice," etc). This is a natural time for such a gathering, as it can mark the conclusion of the formal period of mourning.
However, since unveiling is a purely a minhag (custom), unknown to either the Talmud or the Codes of Jewish Law, there is no halakha (law) to when or how it must be done. While the custom has become to do it within the first year, if for some reason the ceremony is delayed, there is no reason why it cannot take place at a later time. If family and friends find it meaningful and comforting to gather and recall the life of someone who has died, there is no reason to deny them that opportunity for healing.