The timing of the months in the Islamic calendar is based on astronomical observation.
A new month can only begin after a Waxing Crescent Moon is observed shortly after sunset.
An Islamic year consistently falls about 11 days short of the solar year.
For that reason, the Islamic calendar cannot be used for agriculture or other activities traditionally linked to the seasons, and most Muslim countries officially use the Gregorian calendar as their civil calendar alongside the Hijri system. The traditional version of the Islamic calendar requires an authorized person or committee to make an actual sighting of the Crescent Moon to determine the length of each month.
When this happens, the new month will begin a day later, at short notice.