What are do Vietnamese people usually doing during the TET Tet [we only capitalize the first letter of a proper noun; capitalizing all letters means that it is an acronym, for example USA standing for United States of America] holidays (also known as the Lunar New Year) ?

TET Tet is the most important day of the year for Vietnamese people. It is the first January of lunar calendar.  [Explaining when Tet happens is a bit complicated in English -- this is unavoidable.  The sentence I'm pasting after this is from the Wikipedia entry, which I'm sorry to say is about the simplest complete explanation:]  It takes place from the first day of the first month of the lunar calendar (around late January or early February) until at least the third day. At On this occasion, all members of a family often try to gathered together, even if they live far away. They prepare traditional dishes such as 'banh chung,' or rice cakes, 'nem,' or spring rolls, ... etc. [Don't end a sentence with '...'  That is only used when writing speech or leaving out text from a direct quote.  Instead use 'etc.' for a list that could be longer.  It stands for et cetera, which is Latin for 'and other things.'] They decorate their homes beautifully to welcome relatives, neighbors, and friends. In the north, each family will buy a the branch of a peach tree, and but in the south they prefer the 'Mai' flower. These two flowers represent TET Tet, they bring the spirit of the new year. 

During the TET Tet, people wish each other health, happiness, success, and prosperity for the incoming year. The Children are very eager because they will have new clothes, and especially because they will receive red packets by from their grandparents and parents. [What do the red packets contain?  You should explain further.]  We call this action custom 'mừng tuổi,' which means _______. 

In the time of TET During Tet, Vietnamese people avoid to do doing some things, for example: they don't [We usually don't use contractions in formal writing.  'Do not' would be a bit better.] sweep the house because they think that if they do it, they will sweep away all good fortune. Also, each family selects one successful people person to come first at their home enter its home first. This choice is very important, as it that determines the luck, the and success in the new year for the family invited.  

This year, I can't return to my country for the TET Tet. I am so sad, but I plan next year i will to return to Vietnam with to visit [since your family is still in Vietnam] my family. I will call my mother to wish her a happy new year from France.

Happy lunar new year to all!
Chúc mừng năm mới tất cả mọi người ! [To you as well!]

What do Vietnamese people usually doing during TET holidays (Lunar New Year) ?

TET is the most important day of the year for Vietnamese people. It is the first January of lunar calendar. At this occasion, all members of family often gathered, even if they live far away. They prepare traditional dishes such as 'banh chung', 'nem'... They decorate their homes beautifully to welcome relatives, neighbors and friends. In the north, each family will buy a branch of peach tree, and in the south they prefer 'Mai' flower. These two flowers represent TET, they bring the spirit of new year.

During the TET, people wish each other health, happiness, success and prosperity for the incoming year. The children are very eager because they will have new clothes, and specially they will receive red packets by their grandparents and parents. We call this action 'mừng tuổi'.

In the time of TET, Vietnamese people avoid to do some things, for example, they don't sweep the house because they think that if they do it, they will sweep away all good fortune. Each family select one successful people to come first at their home. This choice is very important that determines the luck, the success in the new year for the family invited.

This year, I can't return to my country for the TET. I am so sad, but I plan next year i will return to Vietnam with my family. I will call my mother to wish her a happy new year from France.

Happy lunar new year to all!
Chúc mừng năm mới tất cả mọi người !