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Many Americans celebrate Christmas, which falls on a Monday this year. Because United Parcel Service Inc. (NYSE: UPS) is falling behind in delivery time, people may be anxious about gifts they order very late. December 23 is among the heaviest shopping days as people who are tardy shopping for friends, family and co-workers jam e-commerce sites.

There are a number of reasons people cannot buy the gifts they want to give for Christmas. Among them are that some items have been sold out. While disappointing, it gives shoppers the chance, in many cases, to give alternative gifts. Later delivery falls outside that solution.

Late delivery is not always the fault of shipping companies. Some e-commerce sites promise to have gifts at the last minute, presumably because they are on back order, and the retailers believe they will get them in the nick of time. That is a risky business that relies on both guesses and the supply chain. That supply chain can be undermined by shipping companies, so late delivery becomes a vicious circle.

UPS has warned both retailers and, via news reports, the shopping public that it may not have enough trucks and delivery people to make all delivery dates. Shoppers who procrastinate will have to take their chances. Some people cannot bring themselves to make decisions any earlier.