According to famous Sakhi (Evidence) or story of Guru Nanak Dev Ji, when he was an accountant at a town of Sultanpur Lodhi, he was distributing grocery to people and when he gave groceries to the 13th person he stopped there because in Gurmukhi and Hindi the word 13 is called Terah, which means yours. And Guru Nanak kept on saying, "Yours, yours, yours..." remembering God. People reported to the emperor that Guru Nanak was giving out free food to the people. When treasures were checked, there was more money than before.
The word "Waheguru" appears only 13 times in Guru Granth Sahib, the Sikh holy scriptures and the eternal Guru.
The Vaisakhi which commemorates the creation of "Khalsa" or pure Sikh was celebrated on April 13 for many years.
Ik Onkar also looks like 13.
Bow to NanakSai!
Sai baba let your holy lotus feet be our sole refuge.OMSAIRAM