On Christmas Eve (Sochelnik – Соче́льник), the Christmas service in Russian Orthodox churches starts and lasts until early morning.
On Christmas Eve, the entire family fasts until the first star appears (до пе́рвой звезды́). That's when they sit at the table which features 12 dishes in honor of Jesus's 12 apostles. The feast begins with a sweet porridge called 'kutia' (кутья́) with honey and poppy seed
They then proceed to the main customary foods, which in Russia are traditionally roasted piglet and roasted goose. In the past, people used to make pies, take them to church the day after Christmas to be blessed, and then give them to each other.
Source: RBTH: 10 facts about Orthodox Christmas