Za vikend moja sestra često radi na računaru.

In Serbian, za can be used with time expressions to mean during a period of time. So za vikend literally means during the weekend.

In this sentence, it is understood as a habitual time expression, so the whole sentence means something like My sister often works on the computer on weekends.

A very common alternative is vikendom, which also means on weekends.

After za in this meaning, Serbian uses the accusative.

The noun vikend is a masculine inanimate noun. For masculine inanimate nouns, the accusative singular is often the same as the nominative singular. That is why it stays vikend and does not visibly change.

So:

• nominative: vikend
• accusative: vikend

Moja means my, and it is a possessive adjective. In Serbian, possessive adjectives must agree with the noun they describe in gender, number, and case.

Here, sestra is:

• feminine