There is confusion between the dates of the standardized Bodhi Day of the “real” Bodhi Day.
The standardized Bodhi Day occurs every year on December 8. This is the 8th day of the 12th month. It’s celebrated on this day, every year, mostly in Japan, where it’s a fairly big holiday.
Update on Dec 8, 2022: The next Lunar Bodhi Day is January 18, 2024
The lunar Bodhi Day, like Easter, varies as it’s based on the lunar calendar. It is the 8th day of the 12th Lunar Month. I like to use the Chinese Lunar calendar as the basis for this date since it’s the largest country on a lunar calendar.
A big part of the confusion is that the lunar Bodhi Day for some year, say 2019, usually falls on the next year. That’s because the Chinese New Year pretty much starts on the first new moon of the year, in January or February. So if one Googles “when is bodhi day 2019”, they will get January 13, 2019 – not the answer they really want, which is January 2, 2020.
Further complicating things, because the lunar Bodhi Day is based on time the 12th New Moon starts, that can be a different day for the Eastern and Western hemispheres. For example, for 2019, the 12th New Moon is on December 25, 2019 in the Eastern hemisphere and December 26, 2019 in the Western Hemisphere. Based on those times, the 8th Day of the 12th month falls on January 2, 2020 in the Eastern Hemisphere and on January 3, 2020 in the Western Hemisphere.
In the end, although I live in the U.S., I believe the choice should be based on the time in India where this historic event occurred.
Bodhi Day 