There is more than one way to describe time in Lithuanian. One of them is to use prepositions:

per (during)
prieš (before)
po (after)

Preposition per requires the accusative case:

per pamoką (during a lesson)
per dieną (during a day)
per vakarą (during an evening)

Preposition prieš requires the accusative case as well:

prieš pamoką (before a lesson)
prieš dieną (before a day)
prieš vakarą (before an evening)

And preposition po uses the genitive case:

po pamokos (after a lesson)
po dienos (after a day)
po vakaro (after an evening)

Per pamoką Laimis skambina Dovilei. During the lesson Laimis calls Dovilė.
Po pamokos aš visada važiuoju iš mokyklos. After a lesson I always drive out of school.
Prieš pamoką reikia pirkti pieštuką. Before a lesson it is needed to buy a pencil.
Mes gauname du tušinukus per dieną. We get two pens a day.
Eik į parduotuvę prieš vakarą. Go to the shop before the evening.

There are also some idiomatic prepositions like iš for describing time. The difference is that these prepositions lose their original meaning. The original meaning of iš is from or out of, but it is used in such phrases:

iš pradžių - at the beginning
iš karto - at once