National Public Radio's Buddhists Prepare To Observe Bodhi Day, When Siddhartha Gautama Became Buddha article and podcast begins:
These winter holidays are a time for fellowship, joy and, for religious people around the world, worship. For many Buddhists, Bodhi Day is December 8. And tomorrow starts the celebration. Bodhi Day marks the day that Siddhartha Gautama became the Buddha.
Wikipedia's Bodhi Day mentions Mahayana Buddhism and specifically Zen Buddhism.
For those who recognize Bodhi Day is the Gregorian date of December 8 universal, or do some Mahayana Buddhists use a lunar calendar to place it?