Back in the old days they meant exactly what they say. This being your parcel would arrive on a Monday, Tuesday, Wednesday, Thursday or Friday but never on a Saturday or a Sunday because those latter two were considered none-working days and, for those who can remember, Sunday was a day when almost no shops where open, save for little corner shops that usually opened for an hour or two.
Over time, big and not so big, shops started opening on a Saturday but Saturday never became part of the working day when taking deliveries into account, unless one paid a much higher price on that delivery.
Move into more recent memory and big, and not so big, shops started opening on a Sunday but still, for deliveries, Saturdays and Sundays are not considered Working days when our 3 words are under consideration. The same rules for a Saturday delivered parcel remains in force but Sunday does not get a look in.
So, we now live and work in a week consisting of the full 7 days, but, if you pay for a "Standard delivery" option on a Friday you will still need to wait until the following Monday for that parcel to arrive. However, Saturday has been a standard working day for so many for so long it should now be considered a Standard working day and therefore be covered by our 3 little words.
Then again, if this was ever done the bosses will almost blow a fuse and demand that because Saturday is now considered as part of the standard working week then they should not have to pay their workforce time and a half, or third as some are still paid. But even there there is a work around. Saturday should only be considered as part of a standard working week when taking parcel deliveries in to consideration and should not impact on the wages, payment details, of those who are forced to work on a Saturday.
Of course, there will be some winners and some losers in the above but I, and many others no doubt, would like to see Saturday as part of the Next working day delivery option for no other reason than in this 7 days a week world we now live in demands it be so.