It's been a while since the last constellation! The constellation Leo is one of the most recognizable in the sky, and is obviously one of the constellations tied to the zodiac. Not surprisingly, it is therefor also tied to Hēraklēs.
The lion was considered the King of Beasts by the ancient Hellenic writers. For this reason, Hyginus--our primary source of ancient information on constellations--describes the constellation almost entirely as such, mentioning Hēraklēs in passing only:

"He is said to have been put among the stars because he is considered the king of beasts. Some writers add that Hercules’ first Labor was with him and that he killed him, unarmed. Pisandrus and many others have written about this. " [II.24]

The firs labour of Hēraklēs is, of course, to slay the Nemean lion. You can read about the start of Hēraklēs' life here, but for this constellation, all you need to know is that he was stricken mad by Hera, and killed his wife and children while he was out. As a result, he went on a long journey to cleanse himself of the miasma caused by these killings. First, he visited the oracle at Delphi, who, unbeknownst to him, was whispered to by Hera. The Oracle told Hēraklēs to serve the king of Tiryns, Eurystheus, for ten years and do everything Eurystheus told him to do. Eurystheus gladly provided Hēraklēs with these labors--ten of them, one for each year--and eventually ended up adding two more, resulting in the Twelve Labors of Hēraklēs.

The first of the twelve labours was to slay the Nemean lion. As the lion was terrorizing the area surrounding the mountain, Eurystheus must have seen in the lion a worthy opponent of Hēraklēs, whose tales of bravery and brute strength has proceeded him. He ordered the hero to return with its skin. And so, Hēraklēs went to the cave of the lion after picking up a bow and quiver of arrows, because he was unaware he would not be able to harm the creature with mortal weapons. After a long fight, Hēraklēs eventually chokes the lion, and hails victorious.

The Latin writer Seneca had a beautiful description of the constellation Leo, in the words of our great hero:

"See where the lion, my first toil, glows in no small part of heaven, is all hot with rage, and makes ready his fangs. Forthwith he will seize some star; threatening he stands with gaping jaws, and breathes forth fires, and shakes the mane upon his flaming neck; whatever stars sickly autumn and cold winter with its frozen tracts bring back, with one bound will he o’erleap, and attack and crush the neck of the vernal Bull." [II.942]

I want to add one more thing to he description of this constellation: Hyginus tells another tale about the constellation Leo: the story of Berenikē (Βερενίκη). The name translates to 'bearer of victory', and this version of the story refers to Berenikē II, who lived from about 267 - 221 BC. She was the daughter of Magas of Cyrene and Queen Apama II, and the wife of Ptolemy III, the third ruler of the Ptolemaic dynasty of Egypt. Prior to her marriage to Ptolemy III, she was married to Demetrius the Fair, a Macedonian prince, whom she had killed after he chose to become the lover of her mother Apama. Berenikē was quite the woman, and participated in the Nemean Games even the ancient Olympics. Her own son, Ptolemy IV eventually had her killed, but she was deified and to be honored from that moment on. The following story concerns her, her husband Ptolemy III, and Aphrodite.

"Above his likeness in the sky nearest the Virgin are seven other stars near his tail, arranged in a triangle, which Conon, the mathematician, and Callimachus call the Lock of Berenice. When Ptolemy had married his sister Berenice, daughter of Ptolemy and Arsinoe, and after a few days had set out to attack Asia, Berenice vowed that if Ptolemy returned as victor she would clip off her hair. She placed the lock, consecrated by this vow, in the temple of Venus Arsinoe Zephyritis, but on the following day it couldn’t be seen there. When the king was distressed by this, Conon the mathematician, whom we mentioned above, desiring to win the favor of the king, said that he had seen the lock among the constellations, and pointed out seven stars without definite configuration which he imagined were the lock.

Some authors along with Callimachus have said that this Berenice raised horses, and used to send them to Olympia. Others add that once Ptolemy, Berenice’s father, in panic at the number of the enemy, had sought safety in flight, but his daughter, an accomplished horse woman, leaped on a horse, organized the remaining troops, killed many of the enemy, and put the rest to flight. For this even Callimachus calls her high-souled. Eratosthenes says that she ordered returned to the girls of Lesbos the dowry left to them by their parents, which on one had released, and she established among them right to bring action of recovery."

The constellation Leo is visible at latitudes between +90° and −65°, and best visible at 21:00 (9 p.m.) during the month of April.