Why does the first day start on some day in April? And what is ST?

The first day does not start in some day in april, it starts on 1st january each year.

On 14 April , its the game's birthday, and the year the game celebrates that year, defines the year

 

This might be a bit confusing, but for now thats how it always was. For example today is day 128 (counting 128 from the beginning of the year), and year 16, counting how many times it passed the date of 14 April (years).

ST means server time.

Because everyone is on different timezone, its not relevant what day is for each one, so we use server time to define time of events and such.

Puuuuureeellyy by coincidence does it happen that server time is Romania, GMT+2 , same like mine, just a coincidence, yeah...:))