Could check my grammar mistakes please?

Nothing much wrong here, although the sentences are rather short.

I was invited to a New Year's Eve party or celebration
In the evening, I sat at the computer

It's a load of disconnected sentences. Most of which are correct, but there's no flow. For example the first two sentences would be better as one.

Winter holidays were wonderful, a lot of interesting things happened.
This is wrong.
In the evening, I was sitting at the computer.

Either

In the evening, I sat at at the computer.

Or
In the evening, I was sitting at the computer when.....

I was invited to a New Year's Eve.

A New Year's Eve what, party, ball, séance?

Overall if you just had to write a series of sentences you've done it. If you had to include some sort of narrative, you've not. You described the New Year's Eve party as fun. Then you talk about breakfast, whether it's before or after the party remains a mystery. Then you've got a sentence about sitting on the computer. It's all over the place.

Stick to the party. Who invited you? Where was it? How did you get there? Who did you talk to? Was there food, drink music? What happened?

If you do that you've got some form of narrative.

Izzy, Gamer asks only about grammar

Tho I'd agree as you can see from my response in the other pertinent thread, there's a temptation to inquire of circumstance

Hello,
Some little mistakes are there, otherwise all ok.
Mistakes:
we completed a study on 30th December
I was invited to a New Year's Eve party or celebration
Above little mistake are happened.

What is this supposed to mean? It's nonsense, and anyone, human or bot, that can write a sentence like this has no place advising anyone on grammar.

In the evening, I was sitting at the computer
Correct one is,,

In the evening, I would sit/sit at the computer.