It's May 26 to June 1 here in Vancouver. Was later in June last year. Guess they're worried about low turnout if it's cold and wet :-)

https://bikehub.ca/bike-to-work-week

seriously though, I don't begrudge those who start riding during BTWW... maybe some of them will get hooked! As for the extra traffic on the bike routes... Just more people for me to pass, and they even press the button for me at cyclist controlled intersections