A noun in Russian is in the accusative case when it is the direct object
of a verb.
The direct object in a sentence receives the action of the verb, and
it answers the question whom? or what?
John invited David to a party.
David invited John to a party.
The person whom the subject invited to the party is the direct object of
the verb. In English we can tell who was invited only by the word order:
the direct object follows the verb.
With most nouns in Russian it is possible to tell which noun in a sentence
is the direct object by the ending that it takes, regardless of where it
is located in the sentence.
To answer the question "on what day of the week" something happens, in
Russian we use the preposition в and the day of the week in the accusative case. Examples: В среду мы не работаем.
Мой брат приходит в четверг. В воскресенье они едут в Москву.