People with the noble New Year’s resolution to work out or work out more should mark their calendars: That resolve starts to waver in the third week in January, data shows.

The pattern shows how difficult it is to make a new habit stick.

Attendance is a delicate part of a gym’s business. Gyms need new members but face regulars who complain about crowds, waits and the poor etiquette of January newcomers.

Check-ins on Facebook to facilities with “gym” or “fitness” in the name drop 10% in February, according to two years of Facebook data. The decline starts as early as the third week in January, Facebook researchers say. This dip comes after a more than 50% surge from December as people chase vows to lose weight or get fit.

It takes about 66 days to form a habit, according to a 2009 study by researchers at University College London.

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