MySQL WEEK() function: Does the mode affect average weekly data accuracy?

I currently use the following command to find the weekly average data in a table:

SELECT WEEK(test_date) Week, YEAR(test_date) Year, test_date Week_starting_from_date_shown, AVG(download_kbps) DLWeeklyAvg, AVG(upload_kbps) ULWeeklyAvg
       FROM source_data_dump
       GROUP BY WEEK(test_date,3), YEAR(test_date)  

This will be automated for a program that counts on and on and the week number doesn't actually matter, only the average values MUST be accurate. So the week mode i chose may only be valid for say, this year 2017 but not for 2018 and leap years, so if i leave it as mode 3 all the way will the weekly average data get affected as we move to a new year?