Under the Federal Employees Retirement System (FERS), it is normally best to retire on the last day of a month. If you retire on the last day of a month, your FERS annuity will then begin accruing on the 1st day of the following month, which would be the next day. If you are covered under the Civil Service Retirement System (CSRS or CSRS Offset), it is normally best to retire on the 1st, 2nd, 3rd, or the last day of a month. Your CSRS annuity will then begin accruing the following day. If one of the above days happens to fall at the end of a pay period that would be the ideal situation since you would earn annual and sick leave for that final pay period.
Questions may be directed to your Benefits Contact or AskBenefits@nih.gov.