В каникулы и В каникулах?
Is there any difference between in the meaning в каникулы (винительный падех) and в каникулах (предложный авдеж)? sometimes they use the first one and other time the second

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Answer

со словом "каникулы" используется предлог "на"

— Вам много задали на каникулы?
— Только по литературе. Буду на каникулах читать много книг!

Answer

Russian language uses various combinations of prepositions and cases to express different meanings. For example:
— В дом. Preposition «в» and nominative case of following noun gives direction meaning. This combination used when you want to express direction of movement. It's for into: i'm going into the house.
— В доме. Preposition «в» and prepositional case gives position meaning. We use this combination when what to sey that something is situated inside of something. It's for in. I'm in the house.

The same concerning to «на каникулы» and «на каникулах».
— На каникулы — preposition «на» and nominative case makes direction meaning. It could be used as Faortto said. Literally: How much tasks did the teacher give you towards the vacation. Or: я собираюсь на каникулы = I'm going on vacation.
— На каникулах — «на» + prepositional case = position. When somebody is on (position) vacation we say «на каникулах».

Mind-blowing for you:
— На каникулы я поеду к бабушке. Direction. I'm going to go to my grandmother for my vacation. So it's final purpose to spend time on grandmother place.
— На каникулах я поеду к бабушке. Position. When I'm on vacation I'll go to my grandmother. The difference is that the speaker is going to go to grandmother being on vacation already. From position “on vacation”. May be he will have spent a part of vacation on something and the rest of time he will be on the grandmother.

Answer

Что будешь делать в каникулы?
Что задали на дом на каникулы?
Ничего хорошего не вижу в каникулах. Лучше бы я продолжал учиться...