There isn't a single, universally recognized day in honor of the "sun lord." The concept of a "sun lord" varies across different cultures and mythologies.

Here are some examples:

* In ancient Egypt: The sun god Ra was celebrated throughout the year, with specific festivals dedicated to him, such as the Festival of the Opet and the Festival of the New Year.

* In Hinduism: The sun god Surya is celebrated on Surya Jayanti, a festival that falls on the 14th day of the Hindu month of Magh (January/February).

* In the Inca Empire: The sun god Inti was honored with a major festival called Inti Raymi, celebrated during the winter solstice.

To be more specific, you'd need to clarify which "sun lord" you're referring to.

Can you tell me more about the sun lord you're interested in? Perhaps you're thinking of a specific culture or mythology?