Baldur was the son of Frigga and Odin, and the twin brother of Hod, or Hodur. Baldur's name sometimes appears as Balder, or alternately Baldr. Baldur was beautiful and radiant, and was beloved by all the gods. Hodur, on the other hand, was dark and moody, spent a lot of time in darkness because of his blindness, and was generally unpopular with everyone he met.
In one famous story, after Baldur reveals that he's been having foreboding dreams, Frigga asked all of nature to promise not to cause any harm to her beloved son.
From Sæmund's Edda:“On a course they resolved,
that they would send
to every being,
assurance to solicit,
Balder not to harm.
All species swore
oaths to spare him;
Frigg received all
their vows and compacts.”
Unfortunately, in her haste, Frigga overlooked the mistletoe plant, so Loki – the resident trickster – took advantage of the opportunity and fooled Hod into killing his twin brother with a spear made of mistletoe. Baldur was later restored to life.
Because of the story of his life, death and resurrection, Baldur features prominently in Norse mythology. An important festival was held in honor of Baldur the Good at midsummer, because it was known to be the anniversary of his death and descent into the underworld. Celebrations were held involving big bonfires and outdoor festivities, much of which involved watching the sun rise and set. Bear in mind that in the extreme Northern latitudes inhabited by the Norse peoples, the sun never really sets at midsummer; instead, it touches the horizon and then rises again to begin a new day. When Christianity moved into the Norse countries, Baldur's celebration became the festival of St. John instead.