Guiding had spread internationally in it's very early years, but this was to some extent hindered by World War 1. Following the war, there was an increased desire for international links. The International Council was founded in 1919, and a World Camp held at Foxlease, alongside the World Conference.
It was in 1926 that it was proposed to hold a Thinking Day, and this was ratified in December 1926. As a result, the first Thinking Day was marked in 1927. The date was chosen as 22nd February, being the birthday of the founder, Robert Baden-Powell, and his wife, former UK Chief Commissioner Olave Baden-Powell. It was suggested that each member of Guiding give a penny, cent, pfennig, or the local equivalent, to the Thinking Day Fund. This would be used to support the growth of Guiding worldwide. Units were also encouraged to mark Thinking Day, on their nearest meeting, by both giving thanks for the Founders, and thinking about their Guiding sisters around the world. This was often done through dressing up in international costumes, making crafts or foodstuffs, and learning about culture and Guiding abroad.
In recent years, the World Association of Girl Guides and Girl Scouts (which succeeded the International Council) has released an activity pack for units to work on, with a badge they can buy, to help raise money on Thinking Day. Initially they were based on each of the 'Millennium Development Goals'; after that series was completed, other themes have been utilised.