For ERAS (i.e. regular match, not neuro, neurosurg, ophtho, urology, or ENT):
Request letters in spring/summer of 3rd year
Assemble CV over summer after 3rd year
Write personal statement during summer
ERAS submission allowed around Labor Day
Sept/Oct - ensure all letters have been submitted
Nov 1 - Dean's letter and transcript released to programs
As a rule, earlier ERAS submission = better, especially for competitive fields. Some programs offer interview invitations in Sept/Oct, before the dean's letter is sent out, so the earlier you're ready, the better. Some programs issue invitations on a rolling basis, others wait for the dean's letter (November) and issue invitations in bulk. There's no way to know how programs will behave ahead of time, so get your application in early.
Some people take Step 2 as early as possible, to bolster their application or make up for a poor showing on Step 1. If you did especially well on Step 1, it may be to your advantage to take Step 2 late, so you won't look like an underachiever if you don't get a similarly high score. Some people (like me) are lazy, and we choose to put off Step 2 as much as possible. Since my school does not require any step of the USMLE for graduation, I'm taking Step 2 on Friday, a week after the match.
Good luck!