מדרש על שבת 37:1

"many days": "days" — two; "many" — three. We are hereby taught that peace is offered for two days, and a third day for the battle. And even though there is no proof for this, there is an intimation of it in (I Samuel 30:1) "And David and his men came to Tziklag on the third day." Variantly: From here it is derived that gentile cities are not besieged fewer than three days before the Sabbath, (so that the siege not extend into the Sabbath); but if the siege began, it is not interrupted. And a sea voyage is not begun fewer than three days before the Sabbath. When is this so? For a long voyage, but for a short one, it is permissible.

(We are hereby taught that peace is offered for two days, and a third day before the battle. And so it states: (I Samuel 30:1) "And David Remained in Tziklag for two days." And gentile cities are not besieged fewer than three days before the Sabbath, (so that the siege not extend into the Sabbath); but if the siege began, it is not interrupted. This is one of three lessons taught by Shammai the Elder: A sea voyage is not begun fewer than three days before the Sabbath. When is this so? For a long voyage, but for a short one, it is permissible.)