User blog comment:B214/Volume 23 discussion blog/@comment-92.20.248.68-20170425130247/@comment-31137559-20170425164355

Issei stated that he became a devil in April and Asia became a devil not long afterwards, probably a week afterwards. And Volume 2 was in May. And then Volume 3 and Volume 4 were in June I believe. And Volume 5 started in July and ended at August due to them training for nearly a month in preparation for the Gremory-Sitri Rating Game. There was a considerable timeskip between Volumes 2 and 3. The same goes for Volumes 4 and 5.