Both the PCT and Paris Conventions refer to non-working days and holidays.
Under the PCT
If the PCT national phase deadline falls on a Saturday or Sunday, the national phase entry deadline is the next business day.
See rule see 80.4(b) and 80.5 of the PCT Regulations that says:
80.4 Local Dates
(a) The date which is taken into consideration as the starting date of the computation of any period shall be the date which prevails in the locality at the time when the relevant event occurred.
(b) The date on which any period expires shall be the date which prevails in the locality in which the required document must be filed or the required fee must be paid.
80.5 Expiration on a Non-Working Day or Official Holiday
If the expiration of any period during which any document or fee must reach a national Office or intergovernmental organization falls on a day:
(i) on which such Office or organization is not open to the public for the purposes of the transaction of official business;
(ii) on which ordinary mail is not delivered in the locality in which such Office or organization is situated;
(iii) which, where such Office or organization is situated in more than one locality, is an official holiday in at least one of the localities in which such Office or organization is situated, and in circumstances where the national law applicable by that Office or organization provides, in respect of national applications, that, in such a case, such period shall expire on a subsequent day; or
(iv) which, where such Office is the government authority of a Contracting State entrusted with the granting of patents, is an official holiday in part of that Contracting State, and in circumstances where the national law applicable by that Office provides, in respect of national applications, that, in such a case, such period shall expire on a subsequent day;
the period shall expire on the next subsequent day on which none of the said four circumstances exists.
Under the Paris Convention
The Paris Convention also deals gracefully with such scenarios. Specifically, referring to Article 4, paragraph C.(3):
(1) The periods of priority referred to above shall be twelve months for patents and utility models, and six months for industrial designs and trademarks.
(2) These periods shall start from the date of filing of the first application; the day of filing shall not be included in the period.
(3) If the last day of the period is an official holiday, or a day when the Office is not open for the filing of applications in the country where protection is claimed, the period shall be extended until the first following working day.
(4) A subsequent application concerning the same subject as a previous first application within the meaning of paragraph (2), above, filed in the same country of the Union, shall be considered as the first application, of which the filing date shall be the starting point of the period of priority, if, at the time of filing the subsequent application, the said previous application has been withdrawn, abandoned, or refused, without having been laid open to public inspection and without leaving any rights outstanding, and if it has not yet served as a basis for claiming a right of priority. The previous application may not thereafter serve as a basis for claiming a right of priority.
Note that in Israel Sunday is a working business day while Friday and Saturday are not.