The day after Christmas is known in some parts of the world (chiefly those with a strong British influence) as Boxing Day. A long time ago, servants and tradespeople used to be given a gift (or "Christmas box") on Boxing Day by their employers or customers. When I was a child (in the UK) it was normal to give money to people who had provided a service during the year, such as the people who delivered milk, bread, groceries and newspapers, and also the refuse collectors. Nowadays people tend to buy most things in the supermarket, and I believe that the refuse collectors were banned from asking for money because of the suggestion that anyone who failed to give money mysteriously found their rubbish all over their front garden.
I used to deliver newspapers in the morning before going to school, and what kept me going during the cold, wet, miserable, dark winter mornings was the thought that in the week or two before Christmas I would probably get a tidy sum from my happy customers. These days, if children are still allowed to deliver newspapers, I doubt whether they would be permitted to out on a winter evening knocking on doors and asking for money!
Interestingly, although the tradition of the Christmas box has largely died out in the UK, it is still alive and well in Hong Kong. At Lunar New Year everyone carries around red "Lai See" envelopes containing "Lucky Money", and as well as giving them to children they are also used as a small token of appreciation for security guards, restaurant staff and similar. Bosses also give them to their staff, again as a small 'thank you'.
However, this being Hong Kong, you often see signs around warning you that staff are forbidden to solicit gratuities, and in some cases not even allowed to accept them. This mean-spirited attitude (presumably based on concern about corruption) doesn't seem to make much difference. Security guards in particular are usually very enthusiastic in wishing you "Kung Hei Fat Choi" and on the first few days of the New Year instead of sulking behind the counter they will likely be right there as you walk through the door. I am always happy to give them a red envelope, and they certainly deserve it given the unsocial hours and low pay that is their normal reward.
In restaurants the staff are normally much more discrete, and there is something almost conspiratorial about the way the money is handed over. Again, I think they deserve the money for the long hours and low pay. Plus it can't harm your chances of getting a table for dim sum on Sunday morning.
One thing that used to amuse me about Lucky Money was that you very rarely saw an old $10 note for most of the year, and then they would all turn up in Lai See envelopes, and after that they would briefly be seen in circulation as people spent the money they had received. Then presumably people would start saving them for the next New Year. Putting coins in a Lai See envelope was thought to be a bit naff, though one ingenious alternative was to use US Dollar bills instead. Or Thai Baht if you were even meaner.
Now of course we have the hideous multi-coloured new ten dollar bill (designed by a 6 year-old in a government primary school who won a competition) and you can just go to the bank and request clean banknotes. Where's the challenge in that?