In many cultures, celebrating the fiftieth of an event marks a major milestone like golden jubilees. Their marriage's fiftieth anniversary was a grand celebration.
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Not Just Time
'Fiftieth' can refer to order in a sequence, not just to time-based anniversaries. She finally made her fiftieth sale.
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Ranking Context
Use 'fiftieth' to describe someone's rank or position in competitions or lists. He was proud to be ranked as the fiftieth best player.