Understanding Mode (Optional Parameter , value 0 to 7)
The week can start on Sunday or Monday,
The week of 1st of Jan should have 4 more days to qualify for week No. 1
Otherwise it will be last week of previous year and next week will be No. 1
In the DEMO page for WEEK() try these days to understand the concepts of Mode.
2012-01-01 change the First Day of Week to Monday
2018-01-01 select Sunday as First Day of Week