Definition from Wiktionary, the free dictionary
class reunion (plural class reunions)
- A planned event at which members of a past school class meet.
- The class of 1986 is having its 20th class reunion before too long.
The ordinal number refers to the number of years since graduation, not to the number of reunions. In the US at least, reunions at 5-year intervals are common, particularly 5th, 10th, 20th and 25th.