5 CFR §715.202

1. (a)General. An employee is free to resign at any time, to set the effective date of his resignation, and to have his reasons for resigning entered in his official records.
2. (b)Withdrawal of resignation. An agency may permit an employee to withdraw his resignation at any time before it has become effective. An agency may decline a request to withdraw a resignation before its effective date only when the agency has a valid reason and explains that reason to the employee. A valid reason includes, but is not limited to, administrative disruption or the hiring or commitment to hire a replacement. Avoidance of adverse action proceedings is not a valid reason.