The usual solution in respect of duration of port stay is to allow an appropriate period time to cover loading and discharging, generally called ‘laytime’ or, sometimes, ‘lay days’, which is at the charterer’s free disposal, and to grant him the possibility, in case of need, of detaining the vessel beyond the agreed time, against payment of compensation to the owner for the use of the additional time; such compensation is called ‘demurrage’.
Per Lord Esher in Nielsen v Wait (1885) 16 QBD 67 at p.71:
The days may be described "working days." Now "working days," if that term is used in the charterparty, will vary in diffferent ports; "working days" in the port of London are not the same as working days in some other ports, even in England; but working days in England are not the same as working days in foreign ports, because working days in England, by the custom and habits of the English, if not by the law, do not include Sundays. In a foreign port working days may not include saints' days. If it is the custom or the rule of the foreign port that no work is to be done on the saints' days, then working days do not include saints' days. If by the custom of the port certain days in the holidays, so that no work is done in that port on those days, then working days do not include those holidays. Working days in an English charterparty, if there is nothing to shew a contrary intention, do not include Christmas Day and some other days, which are well known to be holidays. Therefore "working days" mean days on which, at the port, according to the custom of the port, work is done in loading and unloading ships, and the phrase does not include Sundays.
Per Hamilton J in British and Mexican Shipping Co. v Lockett Brothers & Co.  1 KB 264 at p.273:
… a day of work as distinguished from days for play or rest; and I think it is immaterial whether the days for play or rest are for secular or religious reasons, and whether they are so by the ancient authority of the Church or by the present authority of the state.
Per Lord Devlin in Reardon Smith Line v Ministry of Agriculture  AC 691 at p.736:
From this it appears that "working" is a description of a type of day. Prima facie it is a calendar day of 24 hours just as Sundays and holidays are days of 24 hours, which, when excepted, always taken out of the lay days. They are taken out as a whole because, as Lord Sumner said in Love and Stewart Ltd. v Rowtor Steamship Co. Ltd.  2 A.C. 527 at p.537 the exception of holiday is based on days and not on parts of days.