Yalda

Yalda, a Syric word imported into the Persian language by the Syric Christians means birth. It is refered to the "Shab e Cheleh Festival" a celebration of Winter Solstice on December 21st. Forty days before the next major Persian festival "Jashn e Sadeh" this night has been celebrated in countless cultures for thousands of years. In most ancient cultures, including Persia, the start of the solar year has been marked to celebrate the victory of light over darkness and the renewal of the Sun. For instance, Egyptians, four thousand years ago celebrated the rebirth of the sun at this time of the year.
The last day of the Persian month Azar is the longest night of the year, when the forces of Ahriman are assumed to be at the peak of their strength. While the next day, the first day of the month "Day" known as "khoram rooz" (the day of sun) belongs to Ahura Mazda, the Lord of Wisdom. Since the days are getting longer and the nights shorter, this day marks the victory of Sun over the darkness. Fires would be burnt all night to ensure the defeat of the forces of Ahriman.
It is not clear when and how the world "Yalda" entered the Persian language. The massive persecution of the early Christians in Rome brought many Christian refugees into the Sassanin Empire and it is very likely that these Christians introduced and popularized "Yalda" in Iran. Gradually "Shab e Yalda" and "Shab e Cheleh" became synonymous and the two are used interchangeably.
Happy Yalda