The first day of the New Year is also an important date in Polish calendar. Not only because it is the International Peace Day. Poles attribute this day to the name of Mieszko and Mieczyslaw. All men with this first name have their namesday on January 1st. The namesday of Mieczyslaw and Mieszko is on the 1st of January not by a coincidence. Mieszko was a name of the first historic ruler of Poland from Piast dynasty. Mieszko was also the first Polish prince which achieved international recognition since he unified Polish tribes against German invaders. He also adopted Christianity by marriage with Czech princess, Dobrawa.
The name Mieszko is for Poles - what Luis is for French.
Poles had two main royal dynasties - Piasts, which established the first Polish state and Jagiellons who expanded the state into Polish-Lithuanian Commonwealth, one of the biggest, multinational state in Europe.
The name Mieszko - was very popular among Piast princes since it was a name of the founder. See the images of Polish kings - Mieszko I, Mieszko II and Mieszko III, the Old.
For a long time it was believed that Mieszko is an earlier form of Mieczyslaw. At present historians believe that these names were created separately because their meaning is just different.
There are three theories about the origin of the name Mieszko.
Mieszko comes from "Miecislaw" which means "toss a fame" (in Polish - mieci slawe) according to the interpretation that Slavic names originate rather from attributes of character than from war or weapons.
Mieszko could originate from the Polish word of mieszko, misko or mis meaning bear" as a man with a strength of the bear.
Mieszko may originate from the word "Msciwy" (English "vindictive") but this theory is the least popular.
Mieczyslaw as a name was formed later and it means probably "man glory" or "sword glory" from the Slavic element miecz = "sword" combined with slav = "glory".
I recommend this book about Polish history written by Adam Zamoyski and entitled: The Polish Way: A Thousand-Year History of the Poles and Their Culture
I also recommend Polish First Names Author: Knab, Sophie Hodorowicz