The custom, which originally had a social purpose, comes from the USA.
It is also Father’s Day, which was introduced from the USA in 1935.
According to the legend, the Danish flag, Dannebrog, on this day fell from the sky by Lyndanise in Estonia, where King Valdemar II the Victorious was crusading in 1219.
The Danes often meet with family and friends to have dinner together.
If the weather is good, they then proceed to a local bonfire venue. John's Eve All Saints’ Day, which is celebrated on the first Sunday in November, was originally a commemoration day for the dead saints held on 1 November.
When the liberation was announced in the 8.30pm BBC broadcast on , many Danes spontaneously placed lit candles in their windows.
This became a custom that is still kept up by many Danes.
So is the American Halloween where the children dress up as ghosts etc and go around ringing door bells like at Shrovetide.
When the door is opened, they say ‘trick or treat’ (in English).
There are also May Day (Labour Day) and April Fools Day, where Danes tease each other with pranks and outlandish stories.