Article 20 CISG
(1) A period of time for acceptance fixed by the offeror in a telegram or a letter begins to run from the moment the telegram is handed in for dispatch or from the date shown on the letter or, if no such date is shown, from the date shown on the envelope. A period of time for acceptance fixed by the offeror by telephone, telex or other means of instantaneous communication, begins to run from the moment that the offer reaches the offeree.
(2) Official holidays or non-business days occurring during the period for acceptance are included in calculating the period. However, if a notice of acceptance cannot be delivered at the address of the offeror on the last day of the period because that day falls on an official holiday or a non-business day at the place of business of the offeror, the period is extended until the first business day which follows.