Observing festivals is an old tradition. We have to observe that on correct day and follow traditions correctly. Hindu dharmashastras clearly mendates when to observe which festival. Each festival has a specific month, tithi, nakshatra or combination of tithi and nakshatra or sometimes day. Along with it that combination needs to be observed at the specific time on that day. General rule is when that specific time is not specified the tithi prevailing at the sunrise time should be taken, otherwise the day when that month/tithi/nakshatra combination appearing at the specified time / kala should be taken. karwa Chauth Rule as per dharmashastra.
Explantion: "Karwa Chauth to be observed at moonrise (chandrodaya vyapini) and poorvaviddha, best if its tritiya viddha. If visible during both days take the first one, if not visible on both days then take the second one". The specific time for karwa chauth is moonrise time. This means the chaturthi tithi needs to be visible at the moonrise time. Chaturthi tithi appearing at the sunrise time cannot be taken if it appears on moonrise time on either days.
Every year this confusion arises.
Rule for observing karwachauth:
- Purnimant Kartika Krishna paksha Chaturthi which is Amavasyant Ashwin Krishna Paksha Chaturthi
North Indian calendric system uses Purnimant Panchangam in which months end in purnima. In South india, Gujarat, Maharatra Months end in Amavasya. So shukla paksha remains common for both. Only Krishna paksha name differs. This means When there is a Kartik Krishna Paksha in north in south and gujarat it's Aswhwin krishna paksha. According to North Indian System Krishna Paksha comes first and then Shukla Paksha. According to South Indian, Gujarat, Maharatra System Shukla Paksha comes first followed by krishna paksha. Hence we have difference.