Normally, a yahrtzeit is observed on the anniversary (on the Jewish calendar) of the passing. If the deceased was buried two or more days after the death, some have the custom of observing the first yahrtzeit on the anniversary of the day of the burial as opposed to the anniversary of the passing (though in all subsequent years the yahrtzeit is observed on the anniversary of the death).
Others disagree and observe the yahrtzeit on the anniversary of the passing even on the first year—regardless when the burial happened. This is the accepted custom—and our Yahrtzeit Calculator also follows this opinion.
Rabbi Menachem Posner