August 5/6, 910: Battle of Wednesfield/Tettenhall (Mercians and West Saxons defeat Northumbrian Vikings)
A very brief history of the first wave of Viking invasions of England would show them showing up around 790, becoming an increasing nuisance through the ninth century until they start taking over whole kingdoms in the 860s and 870s. Alfred "the Great" of Wessex manages to hold them off in the 870s, and in 880 most of the free armies (the ones that haven't already settled) look for easier pickings on the Continent. Alfred does a lot of fortress-building in the 880s, so that when the Vikings return in 893, it's a different story: the English offer well-organized and mobilized resistance, such that after three years the Vikings give up, and some of them settle already-Viking areas of England and the rest go back across the Channel again.
The next act features Alfred's descendents taking back the Viking-occupied areas and knitting them into a single English kingdom, and it starts with Alfred's son, King Edward the Elder (899-924). We don't know much about the first ten years of his reign, but in 909 he sends an army of West Saxons and Mercians into the territory of the northern army (what had been English Northumbria until 867), and they spend five weeks killing Vikings. The following year the northern Vikings cross south of the river and ravage Mercia, and they are returning home heavily laden with loot when the army of the West Saxons and the Mercians catches up with them. Different versions of the Chronicle record that the battle took place on August 5 or 6, and at Tettenhall or Wednesfield -- the different place-names are easily explained because they're only four miles apart (see map), so the "real" site is presumably near both. The English armies killed very many of the Vikings, and put the rest to flight.
While we may not believe the Chronicle's record that "many thousands" of Vikings were slain, it was a crushing defeat with a very significant effect: in Edward the Elder's campaigns in the 910s to retake the Viking-held lands south of the Humber, the Northumbrian Vikings played no part at all. There are earlier examples of the Northumbrian and East Anglian Vikings joining forces (896 and probably 902), but after the battle of Wednesfield/Tettenhall they seem to have stayed north of the Humber, until the great battle of Brunanburh in 937.