The day I went to Hokkaido in August was the coldest it had been in eighty years.
The day I visited Hokkaido in August, the temperature was the lowest it had been in eighty years.
ーThe day I went to Hokkaido in August was the coldest it had been in eighty years.
「私が北海道に行った8月の日は、80年ぶりに寒い日だった」
the coldest it had been in eighty years で「80年ぶりに最も寒い」
ーThe day I visited Hokkaido in August, the temperature was the lowest it had been in eighty years.
「北海道を訪れた8月のその日は、80年ぶりに気温が最も低かった」
temperature で「気温」
the lowest it had been in eighty years「80年ぶりに最も低い」
ご参考まで！
It was the coldest August day in Hokkaido in 80 years.
「the coldest August day」は「8月では一番寒い日」という意味で、そのまま時期と記録を具体的に伝えています。
例文: When I traveled to Hokkaido, it was the coldest August day in 80 years!
「when I traveled to Hokkaido」は「北海道に旅行したときに」という部分を表しております。
役に立ちそうな単語とフレーズ:
・coldest - 最も寒い
・August - 8月
・day - 日
・years - 年
カナダ 
アメリカ合衆国 
