How do I choose my retirement date?

FERS
FERS employees can retire at age 62 with 5 years of service, 60 with 20, or at minimum retirement age (MRA) with 30. You can also retire under MRA+10 but your annual benefit will be reduced 5% for every year you’re under age 60 at retirement. The annuity starts the month following the month in which you retire. If you retire before October 31st, for instance, your annuity will begin in November.
CSRS
CSRS employees can retire at age 62 with 5 years of service, 60 with 20, or 55 with 30. You don’t have to retire before the first day of the month; you can retire up to the third day of the month and still receive your annuity for that month. That month’s annuity will be prorated for the days you wait.
For annuity purposes, OPM divides the year into 12 30-day months so your monthly annuity payments will be the same. They don’t increase or decrease based on the actual length of a particular month. So, to prorate your first month, the one, two, or three days you wait to retire will be subtracted from 30.