Se celebra, se miran, se prepara, se sirve, and se cena are impersonal or passive expressions, which means that no individual is identified as performing the action. English accomplishes the same deemphasis of the subject by using impersonal subjects, like people, they, you, one, or by using the passive voice (is served).
En el otoño se celebra el día de Acción de Gracias en EEUU. Esa día se miran partidos de fútbol Americano. Se prepara un pavo para la cena que se sirve con relleno. Se cena con la familia.
In fall, people in the U.S. celebrate Thanksgiving. That day they watch football games. You prepare a turkey for dinner that is served with stuffing. One eats dinner with family.
In Spanish the se pronoun is used in place of any specific subject with a third person verb.
The use of se is very common, especially in instructions, generalizations, recipes, and announcements.
Revlexive vs Reciprocal vs Impersonal/Passive