One of the most important (and brutal) events of season 1 is actually one of the better documented historical events in the series. Ragnar Lothbrok's raid on the Northumbrian monastery on the Island of Lindisfarne has substantial historical evidence for its occurence, and is considered by most historians to mark the official beginning of the Viking Age. Placing the legendary Ragnar at the head of that raid was a fanciful convenience for Hirst's story, but the raid itself seems to closely track the historical record.
In 793, Scandinavian raiders (more likely Danes than Norwegians) landed a longship at Lindisfarne and "attacked the sacred heart of the Northumbrian kingdom" (via English Heritage). A contemporaneous account of this raid exists in the Anglo-Saxon Chronicle, an Old English source that provides our most reliable record of the time period beginning with the rise of Wessex and ending in a unified England. This presented a problem for Hirst, however, since he wanted to tell the Scandinavian side of the story. The Anglo-Saxon Chronicle undoubtedly frames most of the events of the time in the light most favorable to the Christian Saxons. While Hirst could certainly use the more thorough Chronicle to check his events and provide information that might serve the historical veracity of his series, in order to tell a truly Scandinavian history, he needed an authentically Scandinavian source. Therein lies the problem.
According to an interview with History Extra, Hirst and his historical advisor on the series turned to a loose collection of fragmentary epic poetry called the sagas to try and understand the Scandinavians' view of themselves. For many reasons, the Norse sagas make for far more dubious historical sources than the Anglo-Saxon Chronicle. They were largely authored in the 13th century, hundreds of years after the events of Vikings. They also resemble myths more than historical records, and many contemporary historians have questioned their historicity.
So some of the events depicted in the series actually happened, by all accounts. The raid at Lindisfarne and the Great Heathen Invasion led by Ragnar's sons almost certainly defined the Viking Age in England. With that said, however, everything Hirst used to fill in the blanks between these events is historically questionable.
