What week is it?

By ISO 8601, the standard most calendars and businesses use: a week runs from Monday to Sunday, and week 1 is the week that holds the year's first Thursday (so January 1 can be in week 52 or 53 of the year before, and December 29 to 31 can be in week 1 of the next). A year has 52 or 53 weeks.

53. A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year.

Some calendars, notably in the US, start weeks on Sunday and count the week with January 1 as week 1. That system can differ from the ISO number by one. Check your calendar's settings: most offer ISO weeks.