Like Jesus’s birthday Buddha’s birthday is a matter of preference. One tradition is that the date is calculated from the lunar calendar. The method starts by finding the the first full moon day of the sixth month. Of course there are multiple lunar calendars to choose from. In China this is the fourth month except in years in which there’s an extra full moon. In those years, Buddha’s birthday falls in the seventh month unless it starts a week earlier.
The Tibetan holiday occurs a month later. Happy April 8.