1.
the year of death, counted as the first year
The starting point of the Buddhist memorial-service counting system: the year a person dies is itself counted as the "first" year. Not a separately observed occasion — funeral rites already take place that year — but the concept explains why the service held one year later is called 一周忌 rather than 二回忌.
The starting point of the Japanese Buddhist memorial-service counting system: the year of death itself counts as the "first" (一回忌), so the service held one year later is 一周忌 (not 二回忌), and the one two years later is 三回忌. 一回忌 itself is not observed as a separate occasion — funeral rites already take place in the year of death — so the term is used mainly to explain the counting method.