Because it's been a while since the last constellation, I'm going to give you not one but two constellations today (also because it's much easier to describe both in one post, seeing as the mythology surrounding these two has mixed throughout the years, so I'm really just making it easier on myself).

The Corona Australis, the southern crown, is located in the southern hemisphere, next to the constellation Sagittarius. As such, it is often linked to that constellation and regarded as the wreath that fell off of the head of one of the kéntauros that particular constellation is linked to. In the same regard, it is also linked to the kéntauros 'Centaurus'.

Corona Australis is most often regarded as a wreath, but in some descriptions and depictions, the Corona Australis is a crown. There is, however, one myth in which the Corona Australis does not depict a wreath or crown: in this myth, the constellation represents Ixion, king of the Lapits, who fell in love with Hera after being invited up to Olympos by Zeus. Zeus decided to test his integrity after He discovered His guest' lust for Hera, but Ixion failed miserably. Ixion was expelled from Olympus and blasted with a thunderbolt for failing his test. Zeus then ordered Hermes to bind Ixion to a winged fiery wheel that was always spinning. Therefore, Ixion is bound to a burning solar wheel for all eternity, at first spinning across the heavens, then into Tartaros. The Corona Australis is this very wheel, with Ixion tied to it.

The most famous of myths associated with the Corona Australis is the story of the birth of Dionysos, and the death of His mother Semele. In this version of the myth, Dionysos is born from Semele and Zeus, and while Semele is pregnant with Him, Hera plants seeds of doubt in her mind about the father of the child truly being Zeus. Semele asks Zeus to reveal Himself to her in His true form, and when He is left with no other option, He does so, killing her in the process. Zeus takes pity on His child, and takes Him into either His thigh or testicle, where He is eventually born from. When Dionysos grows up, He raises Semele to Olympos and grants her eternal life. He also places a wreath into the sky to honor her. The wreath would be made of myrtle leaves, for Dionysos left a gift of myrtle in the Underworld in return for His mother, and the followers of Dionysos wore crowns of myrtle.

Interestingly enough, Roman author Hyginus ascribes this myth to the Corona Borealis, and adds that Dionysos first received this wreath--now a golden crown--from Aphrodite. He gave it to His beloved Ariadne, daughter of Minos. According to Hyginus, it was the light that reflected off of this crown, that led Theseus from the labyrinth after slaying the minotaur. Hyginus then goes on to retell the story above, and says that Dionysos did not want to take the crown into the Underworld in fear of it being contaminated with miasma. When He returned from the Underworld, he threw the golden crown into the sky as a memorial.

Hyginus also gives another account for the the northern crown, the Corona Borealis:

As you can see, there are many myths surrounding the two crowns. The Hellenic poet Aratus named them 'Stephanoi' (Στεφάνοι) around 300 BC, but did not differnetiate between the two, although they are not placed close together. As such, the Stephanoi are visible and best viewed at different times throughout the year. The Corona Australis is visible at latitudes between +40° and −90°, and best visible at 21:00 (9 p.m.) during the month of August. The Corona Borealis, located near the constellations of Hēraklēs and Boötes, is visible at latitudes between +90° and −50°, and best visible at 21:00 (9 p.m.) during the month of July.