21 days from today

Three weeks from today

21 days is exactly three weeks, so it lands on today's weekday. With no holiday in between, those three weeks hold 15 business days, which is why "21 days" and "15 working days" describe the same stretch on most calendars, and drift apart around holidays.

Worked example: waiting for a tax refund

The IRS says it issues most refunds for e-filed returns with direct deposit in less than 21 days, though some take longer. If your return was accepted on Monday, February 1, 2027, 21 days later is Monday, February 22, 2027. Weekends and Presidents' Day on February 15 sit inside that span, but the count is in calendar days, so they don't move it. If something else you're waiting for is quoted as "15 business days", the same three weeks on paper, the holiday pushes it to Tuesday, February 23.

The 21-day habit myth

"It takes 21 days to form a habit" is a popular line, usually traced to Maxwell Maltz's 1960 book Psycho-Cybernetics, which described people adjusting to changes in about three weeks. When researchers at University College London followed volunteers building a daily habit (Lally and colleagues, 2009), it took an average of 66 days for the behavior to become automatic, and anywhere from 18 to 254 days. A 21-day challenge is still a handy unit for a goal: long enough to see progress, short enough to finish. Just don't be surprised if it takes another six weeks to stick.

Other three-week counts

• Project sprints and course modules that run for three weeks.
• 21-day notice periods and response windows in contracts and official letters. Check whether they count calendar days or working days.
• Delivery and lead times quoted as "up to three weeks".

Four weeks instead? See 28 days from today.