My Father died in WW ll. We held "Yarzait" on Sivan 20. the date when most hungarian Jews perished. Last week I found out the date and place in YAD VASHEM, of when my Father died. The information was collected actualy to list the date and place of the death of Hungarian Soldiers. My Father was in "Munkatabor" and as such they accompanied the Hungarian Army. I don’t know how reliable the data is. Neither is there a grave. The place where he died Kolomea had at one time a sizeable Jewish population. The Cemetery and records of the Kehila were destroyed. Should I now say Kaddish on the date of the new information that I have?
Say kaddish on the basis of the best information available to you. If you feel that none of the information you have is trustworthy- Yom HaKaddish HaKlali for those whose Yahrtzeit is unknown is the 10th of Tevet.