[译] 他们在圣诞节那天举办了一次晚会。
[误] They held a party at Christmas.

[正] They held a party on Christmas Day.
[注] at Christmas是"圣诞期间"(= during the seasons of Christmas)。"在圣诞节那天"应该用 on Christmas Day. 另外，"在圣诞节前夜"是 on Christmas Eve.。