Lathandar

Lathandar is the most commonly worshipped sun-deity across Erden and is known by many names. She is an important deity in both the Lumeric Pantheon where she is known as She Who Weaves or the Lady of Light, and as a god elsewhere in the world in her own right. Lathandar is a popular deity especially in Barovia and Paxtria, where she is referred to as the Morninglord or Andar, respectively.
Like Kelemvor, the Oroboros, and Tyr, the exact nature of Lathandar differs drastically between cultures. Nonetheless, a few generalizations can be made about Lathandar. She is universally regarded as a sun goddess (or god), associated with birth, light, healing and is generally considered benevolent and even loving.
In Lumeris, Lathandar is easily the most popular deity, where her beauty, openness, and association with light mesh are engrained into Lumeric culture. The faith of Lathandar in Lumeris is organized as a state priesthood called the Golden Order. Although Lathandarites have a reputation for tolerance and xenophilia, a hardline sect of the Golden Order called the Goldmask Fundamentalists has sprung up, aligning themselves with fundamentalist Kelemvorites in their hatred for all things undead (or non-Lumeric, for that matter).

Holy Books & Codes

The tenets of Lathandar were inscribed in the Golden Plates of Gilderion, known colloquially as "the Plates". These are a collection of disc-like golden sheets inscribed with miniscule High Lumeric script, the contents of which have been enscribed in a holy book, also known as "the Plates". The location of the original Plates have long been lost.

Tenets of Faith

Like the sun, thou shalt foster life.
Like the light, may you always illuminate truth.
Like the summer, may you always bring warmth.
Like the morning, may you banish the darkness.

Holidays

The Solaria
The summer solstice, or Solaria, is the principal holy day of Lathandar in the Lumeric calendar. On the Solaria, mead and honey flow, friends and lovers are encouraged to divulge their secrets to each other in the spirit of honesty, and it is customary for gold dust to be sprinkled on the roads of Lumeris. More debaucherous activities often take place in the waning hours of the Solaria, as the combination of liquor, truthfulness, and the spirit of life give way to hedonism.

Lord or Lady?
The gender of Lathandar is a topic of some debate. Generally, the eastern followers of Lathandar, their faith stemming from the Nine Kingdoms of Lumeris, depict her as a female entity. The Paxtrian tradition holds that Lathandar is a male, referring to her as the Morninglord - this conception of Lathandar also holds in Barovia and Lornham.

Above: A typical depiction of Lathandar as the Morninglord in the Paxtrian tradition, as found in the Krezk abbey.
A Dalliance with Tyr? 
Tyr is often seen as quite separate from the Lumeric Pantheon. Yet there are some scholars who profess that Tyr and Lathandar once had a romantic relationship - a belief that is heretical, of course...