The Rebbe advises completing Mishnayot by the end of Kaddish and a tractate on the yahrzeit. He permits visiting the father’s grave while keeping distance from the mother’s. For prayers at graves, he suggests consulting local Rabbanim and blesses the recipient with success in Torah and Yirat Shamayim.
https://www.torahrecordings.com/rebbe/004_igros_kodesh/adar/917