When is it appropriate to visit the graves of one’s parents?
Anytime, especially on the day of the Yahrtzeit.
The custom is not to visit at night.
And also, not during the first year of the passing.
Additionally, there are certain dates in the calendar when visiting the cemetery is not allowed/appropriate, e.g.: Shabbos, Yom Tov, Chol Hamoed, month of Nissan, and Rosh Chodesh.
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Is it better to go Erev Rosh Hashona or after?
Both are okay.
You may go before. There’s a custom to visit Kivrei Tzadikm before Rosh Hashana.
Going after is also fine.
(Some have the custom of visiting deceased relatives on Erev Yom Kippur. This is not a Chabad custom).