It used to be a big event for me when I was younger, but I no longer have any interest about it. It used to be a big event for me when I was younger, but I no longer have any interest about it.

I'd rather say "... I am no longer interested in celebrating it."

It used to be a big event for me when I was younger, but I'm no longer interest abouted in it. It used to be a big event for me when I was younger, but I'm no longer interested in it.

"Interested about it" might be grammatical, but "interesting in it" is the far more common and natural expression.

It used to be a big event for me when I was younger, but I am no longer interest abouted in it. It used to be a big event for me when I was younger, but I am no longer interested in it.

I feel happy at every weekends rather than Christmas, honestly. I feel happy at every weekend rather than Christmas, honestly.

A better word order would be "I honestly feel happy..."

I feel happy at everyier on weekends rather than Christmas honestly. I feel happier on weekends than Christmas honestly.

(1) I'd suggest "happier" rather than "happy", since you're making a comparison. (2) We use "on" and not "at" with the word "weekends". (3) Alternatively, if you want to use "every": "I feel happier every weekend than on Christmas honestly."

I feel happy at every weekends, rather than just Christmas, honestly. I feel happy every weekend, rather than just Christmas, honestly.

Note about "at the weekend" - this phrase would be okay in UK English when discussing one particular weekend, but would not be used in America. When you're writing about "every weekend" though, it's best to leave "at" out regardless of where you are writing/speaking.