His birthday party was coinciding with the school sports day.

coinciding 是动词 coincide 的现在分词形式。coincide 由前缀 co-（表示"共同"）和词根 -incide（来自拉丁语 incidere，意为"落下、发生"）组成，意思是"同时发生、巧合"。 在句中表示"生日派对和运动会这两个事件同时发生"的情况。