Buddha achieved enlightenment on the same date as his birthday, the eighth day of the fourth lunar month, celebrated all over the Buddhist world. The Buddha's Birthday is celebrated a week later in some countries, as here in Vietnam.
Please note: this holiday occurs at the May/June full moon, and sources sometimes differ as to the precise dates. Some sources give the date of Vesak as a month later in leap years. It's worth double-checking with the tourist office before you travel.
Event details can change. Please check with the organisers that the event is happening before making travel arrangements.