Describe the failure precisely
Does the date fail at midnight? Does the quickset not work? Does the day change while the date does not? The exact behavior narrows the diagnosis.
Avoid forcing a binding quickset
Many movements have a calendar-change window when manual quickset should not be used. If the mechanism feels wrong, stop rather than applying more force.
Hand timing can create apparent calendar problems
A date changing at noon may reflect incorrect hand installation rather than a broken calendar mechanism.
Parts depend on caliber and configuration
Day wheels can vary by language and movement version. The caseback or movement information is useful.