How to understand this code, what do the numbers mean?

Question

Java

// Вычисление дня недели для 1 января
        int week = (year + (year - 1) / 4 + 6) % 7;

Answer 1

Is it really easier to write a post here than to google it?
For example

Answer 2

This is obviously the formula for calculating "the day of the week for January 1st".
Substitute the year, for example 2021 - you get the number 5 - i.e. The first of January this year was on a Friday.