Wis. Stat. § 181.0807: Resignation of directors.

(1) Notice. A director may resign at any time by delivering written notice to the presiding officer of the board or to the president or secretary.

(2) When effective. A resignation is effective when the notice is received unless the notice specifies a later effective date. If a resignation is made effective at a later date, the board may make the effective date earlier and fill the pending vacancy before the effective date if the board provides that the successor does not take office until the effective date.