Baldr is a God of Summer, where his brother Hodr is a God of Winter. When the brothers would fight and bicker, their half-brother Thor would set them straight. This is why thunderstorms rage strongest when cold air meets warm air. When Baldr was born, the Gods of Asgard loved him, for what is not to love of a gentle summer's breeze, or long days? Frigga, the mother of Baldr, loved her son so much that she made all things in Creation swear not to harm him; all but mistletoe, which had not grown yet.
Everyone loved Baldr, and reveled that nothing would bring his Summer to an end.
Only not all enjoyed Baldr's longevity. His brother, Hodr, watched with dismay as the crops failed. Sleep would not find him, as the nights grew sweltering and suffocating. Food spoiled all too quickly, and no comfort could be sought from the sun.
Loki, the sly one, saw this as well. He saw Midgard suffering, even as nothing could set things right. Not all the oceans, nor all four winds, not even the ices that grow at the tips of the world. None of the Gods, blinded by adoration, could see what was being done to the world. Something had to be done.
One day, as the Gods were playing contests with Baldr, Hodr sat glowering in his cup. Loki sat next to him, merry as the rest of the Gods. "Is this not a marvel?" He asked the dour God. "Thor can bounce his hammer off your bright brother's forehead, and he will not even suffer a headache! The strongest weapon of all the Gods doesn't leave so much as a bruise!"
Hodr scowled. "It is not a marvel. My beer spoils. I have not slept in days. It is more a marvel that Sif's golden hair does not melt!"
Loki smiled sympathetically; Hodr was the right agent. "Oh, surely it is not so bad as all that." Loki said, though sweat beaded at his neck. "You only need to let some of that frustration out!" The sly God procured a green arrow from his sleeves. He had seen Frigga take oaths from all things, had them swear the most sacred of vows to her, that they would not - could not - harm her son. He had seen the one thing that could not swear, being so young that it did not have a voice. "This is a special arrow, my dear fellow," Loki explained, "that does not need a bow to fire. You must only point it at your target, give it a push, and off it flies!"
Hodr took the arrow, eyeing it suspiciously. "It will not hurt my brother?"
"Of course not! You saw that all manner of weapons swore to your mother, that they could not and would not harm him. You may let fly as many times as you need to, until you are so exhausted that sleep will find you easy, and warm beer will taste freshly brewed."
Hodr went to the Gods at their game for his turn. Baldr smiled at his brother, not tired at all. His brother grasped the green arrow. "Tonight, I will finally sleep." He said, before letting the arrow fly. Only this time, Baldr was not unharmed. The arrow buried into his breast, halfway up the shaft. It was made of mistletoe, you see. The Summer God fell, drops of white blood falling on the arrow as berries, and it began to grow and flower. Mistletoe cannot live on it's own, after all, and requires just a little death from whatever tree it is in.
As Baldr died, so did Summer wane. The Gods saw then what was going on in Midgard, and hurried about their tasks. All but Odin, Frigga and Hodr. They mourned for Baldr. Frigga stopped at her loom, and the clouds grew lurid and grey. Odin rode in a frenzy to Hel, to plead for the spirit of his son. Hela, the Goddess of that Realm, would only give him back should mischievous and dangerous spirits that broke free were returned to her. The Allfather agreed, and rides all Winter to gather these spirits, and ransom for his son's return.
Hodr slept, but it was not a good sleep. He slept cold with guilt and sadness, though he knew that Loki had tricked him. The brighter that Baldr shines in life, the colder and deeper is Hodr's sorrow.