Supernovas are big, impressive events, but rare enough that they are seldom observed without the aid of a telescope. The brightest one in living memory was SN1987A, which occurred in the Large Magellanic Cloud and was visible from the Southern Hemisphere in February of 1987. It got as bright as magnitude 2.9, which puts it a bit dimmer than Polaris but easily visible to the naked eye. There have been much brighter ones, though. The most famous is probably SN1054, which created the Crab Nebula and is well documented as having been visible in daylight, likely reaching magnitude -6 and persisting for weeks.
But the brightest on record was SN1006. Believed to have been a Type 1a supernova (where a white dwarf explodes after robbing a companion star of enough material to pull itself over the Chandrasekhar Limit) at a distance of just 7,200 light years, this is estimated to have had a peak magnitude of -7.5, which is brighter than a crescent moon, and bright enough to cast shadows if you look closely. Certainly bright enough to be plainly visible in daylight, and many observers recorded it, saying it persisted for months in the sky and appeared large, not just a point of light, perhaps a quarter the size of the Moon. The Egyptian Ali ibn Ridwan recorded that it exceeded “the light of the Moon when one-quarter illuminated” and appeared low on the southern horizon. Chinese records mention the new star in May and again in December in the constellation Di, which is between the Western constellations of Lupus and Centaurus. Monks at the Abbey of St Gall in Switzerland recorded the event as well, writing that its apparent size varied, that it was below the plane of the ecliptic, and was visible for at least three months. Someone controversially, there is also a Hohokam petroglyph in Arizona which may depict it. And a Yemeni report was uncovered last year also describing the event.
And now we have another! A new translation of work by the medieval Persian scholar Ibn-Sīnā (also transliterated as Avicenna; you may have heard that name before) has revealed another observation. Ibn-Sīnā’s “Book of Healing”, one of many books he wrote on medicine and philosophy, surprisingly contains a description of the 1006 supernova. Like the Swiss monks, he described it as persisting for about three months, and changing in size and shape. It’s unclear whether it was actually changing in appearance, by the way, but it is possible that it was close enough for observers to make out light echoes in a surrounding nebula. Absent a time machine, this is of course the best we’re going to get. But we can also look and see what it’s like now. Here’s SN1006, a thousand years later:
Calli Arcale's Fractal Wonder 