Big 4 accounting firms recruit most of their first-year staff through campus recruiting. Most candidates receive their offers through either first round or second round hires.
First round candidates usually intern in the summer before graduation. At the end of the summer internship, they are given an official “start date” which is usually 12-18 months later, that is, anytime from September of next year to January in the year after.
The second round of new hires are usually given offers in the months leading up to their start dates. Start dates usually coincide with the start of academic semesters. Big 4 tend to hire “way out”, i.e. hiring people many months in advance of the intended start dates.
The above Big 4 hiring process applies only for “new hires”, i.e. those without full-time experience. Professional hires are made at any point during the year when there is a need.
The firm typically mails out, via FedEx/UPS, a formal offer packet which includes the terms of the offer. HR will follow-up that packet with a phone call to ensure the new hires received it and ensure they don’t have any unanswered questions.