Today is Friday the thirteenth. There are many things that are related to the number thirteen and here's one of them.
UrbanDictionary.com
In the Medieval Ages there was a period when bakers began cheating the public at such a rate that public outcry reached the ears of several kings. As bread was a daily staple of Medieval life, the bakers knew that they could charge a lot of money for minimal portions of their products. As such, kings levied laws against bakers stating that they were to lower their prices and keep honest. In fact the common term "A Baker's Dozen" (meaning 13 instead of 12) came from this time period. Any baker caught selling less than an even dozen was strictly and harshly punished. As a result bakers began adding one extra loaf to be certain their count would be correct or even over the amount decreed by law.
It's hard to say where thirteen became known as a "baker's dozen" but here is the largely accepted story in two versions from mentalfloss.com and urbanlegend.com
The next time you get the gift of an extra doughnut added to your dozen, thank crooked 13th-century bakers for the bonus treat. In the 1260s, British bread makers were notorious for shorting customers with skimpy loaves. King Henry III was so irked by the problem that he implemented a new law to standardize the weight of a loaf—selling puny loaves could result in beatings or jail time. Since bakers wanted to stay on the right side of the law, one common trick was to give 13 loaves to any customer buying a dozen. Even if the loaves were light, the extra would cover the shortfall. It was an easy fix for bakers, and since low-carb diets were still seven centuries away, customers rejoiced.UrbanDictionary.com
In the Medieval Ages there was a period when bakers began cheating the public at such a rate that public outcry reached the ears of several kings. As bread was a daily staple of Medieval life, the bakers knew that they could charge a lot of money for minimal portions of their products. As such, kings levied laws against bakers stating that they were to lower their prices and keep honest. In fact the common term "A Baker's Dozen" (meaning 13 instead of 12) came from this time period. Any baker caught selling less than an even dozen was strictly and harshly punished. As a result bakers began adding one extra loaf to be certain their count would be correct or even over the amount decreed by law.