。
She thought back on her first day of school.
Literal
She [topic-は] school [target-に] went first day [object-を] recalled.
学校に行った最初の日 is a relative clause: '[the] first day [she] went to school' modifies 日 ('day'). The past 行った is in attributive position — Japanese routinely uses past forms inside noun-modifying clauses to describe a completed action embedded in a noun phrase. 最初の日 ('the first day') is the standard phrase for a debut day, often loaded with first impressions and emotional weight. Memory of one's 入学初日 ('first day of school') is a recurring narrative trope in Japan — particularly the first day of elementary school (小学校入学), with its standardized backpack (ランドセル), anxious parents, and ceremonial start.