2021 first long break

2021 First Long Break
--Titles usually have the first letters of each of their words as capitals.

My job has 12 days’ pay-off-day.

I'm not familiar with the noun expression "pay-off-day".
A possible alternative sentence could be: "My job has 12 days' paid leave." or: "My job has 12 days' leave with pay." or "My job comes with 12 days' holiday pay."

The company also has a common rule that we should take off 4 or more days in a row.

So, my first long break is 5 payed days plus 2 public holidays.

So, my first long break is 5 paid days plus 2 public holidays.

In that 7 days I have worked 6 days for a part time job.

Your sentence is ok. However, consider also:
"In that 7 days, I worked 6 days for a part time job." Also:
"In that 7 days, I worked 6 days for a part-time job."

In that 6 days I got so familiar with most colleagues.

In that 6 days, I got to know most of my colleagues well.

I also (first time in my life) watched a young life passed away.

"Also, for the first time in my life, I watched a young life pass away."
It's usually a good idea to avoid using brackets if another way can be found to express your ideas.

I managed to fully complete 2 sets of test paper!"I managed to fully complete 2 sets of test papers!"¶
A set usually means that more than one item makes up a group of things. ¶
Consider, "I managed to fully complete one set of test papers." This sounds better than "I managed to fully complete one set of test paper." So, it is probably better to say "I managed to fully complete two sets of test papers." rather than "I managed to fully complete 2 sets of test paper!"¶
Please let me know if you would like further help with this.

"So, during this first long break I make a lot of achievements.achieved a lot."¶
¶
Special note: The phrase "a lot" is used a lot in everyday speech and writing!! However, there are other phrases you could use if you were looking for a bit of variety. For example, you could use "a great deal" or "much" or even "more than I expected".¶
--> "So, during this first long break I achieved a great deal."¶
--> "So, during this first long break I achieved much" OR "So, I achieved much during this first long break."¶
--> "So, during this first long break I achieved more than I expected."

The most person I need to say “Thank you” is me!

The person I most need to thank is myself!