The Ugaritic version of the Myth of Baal and Anath, the bull god and the fertility goddess, dates from the Bronze Age along the Mediterranean coast of Syria. There are many variants of this myth cycle retrieved and translated in archaeological excavations from the islands in the Aegean, Turkey, southwards to Israel, and across North Africa.  This myth probably had roots there in the early Neolithic period, and has been interpreted as a  metaphor for the annual weather and agricultural cycle along the eastern shore of the Mediterranean.  Henna, a plant indigenous to this area, was used by women in two parts of this myth cycle.  Women hennaed their hands in connection with a spring fertility/sacrifice festival, and the Goddess Anath hennaed  in connection with a harvest/victory celebration  These times were appropriate for henna use, as henna put out new growth in the spring as the season warmed,  and when the rains returned at the end of the drought in the fall.

Baal, the Bull God, was identified with thunder and life-giving rain.  Anath, his sister and consort, was a fertility goddess and a Virgin Warrior Goddess.  In myth cycle, Baal was annually killed by Mot, the god of summer heat. When Baal was killed, the rains stopped through the summer drought.  Mot scorched the earth, send dust storms, and the earth became hard and cracked.  When Anath brought Baal back in the early autumn, the rains returned, and plant life recovered, and it was possible to plow the earth again. 

In the eastern Mediterranean, rainfall was scant between April and October.  If grain, especially barley, was planted too early or too late, there would be no crop.  If it was harvested too late, rain would rot the crop.  Coordinating the barley planting and harvesting cycle with seasonal rainfall was crucial. If rains were sparse or failed, the crop would fail, and there would be famine.  Conceptualization of the tension between summer droughts and life-giving rainfall was expressed in myth, as a battle between the god of rain, Baal, and the god of withering heat, Mot. The myth cycle is interpretable as a metaphor for  the likely percipitation, drought, wind, and heat through the year, and included seasonal festivals for planting, harvesting, fishing, sacrifice, death and rebirth.

Observation of the annual climate cycle was crucial to grain agriculture.  Grain must be planted and harvested at the appropriate times to yield a crop.  Along the eastern coast of the Mediterranean, henna shoots out new leafy growth at the warming time in the spring, before the end of the rains, and again at the return of rain in the fall. This fresh henna growth stains skin vibrant red-orange. Other times of the year, the plant may go dormant during chilly winter weather and hot, arid weather.  If a person noted when the henna plants had put out new growth, with good stain, and harvested or planted then according to the myth cycle, that crop would be likely to succeed.  Therefore, henna was a useful indicator for these agricultural activities. 

At the vernal equinox, young women hennaed their hands and went to seek their husbands at a sacrifice and fertility festival.  At this festival, Baal disappeared, was sacrificed, or was murdered by Mot, the rains ended and the earth was plowed for the last time.  Anath and humananity  grieved for Baal.  Through these summer months, the increasing heat scorched the earth, the soil cracked, and Mot ruled.  People prayed for the return of Baal, rain, so the earth could be green again. Anath sought her brother and consort, Baal, to bring him and his rain back and save the earth from famine. 

Baal was found brought back in the autumn, first with thunder, then with the seasonal rains.  In the myth cycle, a great celebration was held.  Anath prepared herself for this celebration by hennaing her hands, kohling her eyes, braiding her hair and putting on her finest adornments. She went into the palace to the celebration and closed  the doors behind her. She then slaughtered Mot and all the enemies of Baal.  She beheaded Mot, then beheaded all of his dust-colored followers.   The warrior goddess threw their heads across her back, and stashed their arms and legs into her waist sash, and continued until she waded through their bodies up to her knees. 

If this slaughter can interpreted as a metaphor for grain harvest, the warrior goddess Anath was not committing vengeful carnage, but was a woman who gathered grain before the winter rains set in.  She went to her field to cut off the heads of the grain, and threw those heads into a sack on her back for later winnowing. .  She cut and stashed the straw from the grain into her sash, so that could be fed to her cow, sheep and goats.  She hennaed her hands before she went into the field.  Neolithic women harvested grain with a jawbone, later generations used a stone knife or metal the sickle.  All of these hand harvesting techniques were rough on hands, callousing, cutting and blistering them.  Henna strengthened the skin on her hands, and kept callouses firm and strong  so they wouldn't crack and bleed.  Henna put out new growth at the end of the summer drought.  If that woman watched the henna bushes for new growth at the end of the drought, she'd know when to harvest, and the henna would be there, on the way to the fields, to crush and apply to her hands, to make the harvest a little easier for her.

The earliest traditions of henna seem to have grown from the coincidence of henna's seasonal growth patterns in the region where grain agriculture originated.  Henna provided people with a useful marker for the agricultural cycle, as well as a dye that strengthened skin for harvest work.

References:

Cassuto, U.
The Goddess Anath, translated from the Hebrew by Israel Abrams
The Magnes Press, The Hebrew University, Jerusalem 1951

De Moor, Johannes C.
The Seasonal Pattern in the Ugaritic Myth of Ba’lu According to the Version of Ilimilku
Verlag Butzon & Berker Kevelaer, Neukirchen – Vluyn, 1971