pre-noun adjectival
Used almost exclusively before nouns referring to time, especially 日 (day), 朝 (morning), 年 (year), etc. It is a pre-noun adjectival (rentaishi) and cannot stand alone.
明くる日、彼は早く起きた。
The next day, he woke up early.
あくる朝、雪が積もっていた。
The following morning, snow had piled up.