User blog comment:Crimson and Darkness/Volume 25 discussion blog/@comment-26952857-20180419020450/@comment-3965848-20180419141045

@Sbh1fr The story starts roughly at the beginning of Issei's second year in high school in April, Vols 5 & 6 are during summer vacation, V18 is during Christmas, V19 is New Years and by V22 is the start of Issei's third year in high school.