The Pantheon is a huge spherical sundial, where every day at noon the sun enters through the dome's oculus and creates a Circle of Light that illuminates the side towards the portal.
The Circle varies in height depending on the season and creates a series of illuminations, the most important and spectacular of which is the Arc of Light, discovered by Marina De Franceschini in 2014.
April 21 was the Dies Natalis of Rome (Rome’s Day of Birth), thus also an important date for the Pantheon.
The Circle of Light illuminates the great bronze portal for fifteen consecutive days, from April 15th to 29th.
The same illumination is also visible four months later, from August 13th to 27th, days that are not linked to any particular feast in the Roman calendar (except August 15th, Feriae Augusti).
After the Spring Equinox, the days lengthen until the Summer Solstice on June 21st. The Sun rises higher and higher at midday, and the Circle of Light descends toward the pavement.
When the Arc of Light is visible (April 6-7-8), the Sun has an inclination of 55°; on April 21st (Dies Natalis), it reaches 60°, and at the Summer Solstice, it reaches its maximum vertical altitude of 72°.
This and other spectacular illuminations such as the Arc and the Square of Light with their symbolic meaning are explained in detail in the book by Marina De Franceschini and Giuseppe Veneziano: «Pantheon Architecture & Light» (English edition also), with splendid unpublished photographs.