Y we say... It was one of my best day..
Instead of...it was one of my best days
According to your profile, you are an English teacher. Whilst, of course, we realise that a non-native speaker can teach English (some to an almost perfect degree), and it is comprehensible that you might think that "It was one of my best day" might be correct, I am 100% certain that you know that "Y" is not an English word.
I hope that you are teaching your students that it is very important to use only standard, complete English words.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.
Many people are influenced by the short-cuts used in messages sent over mobile network.
My wife even says a person using short-cuts in messages is up-to-date, I definitely don't agree with her.