On Saturday mornings it is often to see people on the street carrying a bag with stickers. These people are doing volunteer work and this is called "Flag Day".
Flag Day in Hong Kong is a day on which public fundraising of charities takes place and is approved by the government. It is held on almost every Saturday mornings between 7am until 12:30pm and selected Wednesday mornings.
Volunteers with some stock of "flags" and a bag would stand on streets to raise funds from pedestrians. Back in the days actual small flags were handed out to people who donated money. Nowadays, people would receive a little sticker instead, which typically bears the name and logo of the beneficiary, to place on their clothing after they donate money, as a note to other volunteers that the person has made donations.
People usually donate with coins, but actually there is another hole in the bag which allows bank notes.
Some approved flag day organisers will also use Octopus card readers on their allocated flag days with the pouches bearing the name of the respective organisations and the statement "Approved by Social Welfare Department."