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.
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. I write article every year explaining this. Previous year Karwa chauth article Rule for observing karwachauth: Nirnaysindhy says:
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.
Now what most people, online site do is take the tithi visible at the sunrise and they think karwa chauth is always fourth day from the Purnima, is wrong from the above quote dharmashastra from Nirnaysindhu.
In 2009: Average moonrise time in india and america varies from 8 PM to 10 PM (depends on the location). The moon should be visible when chaturthi tithi is rising. If it doesn't happen on both days THEN only take the rising tithi. If the tithi changes to chaturthi just before moonrise then that day should be taken no matter even if tritiya is in morning or not.
Please visit http://www.mypanchang.com/panchang2009.html to visit panchang for your city to find moonrising time for your city. In India tritiya tithi finishes at 9:46 AM on October 7th 2009 and then Chaturthi tithi comes and it persists till next day morning on October 8th till 8:28 AM but not long enough to hit moonrise time on October 8th 2009. Hence Karwa Chauth in India is on October 7th, 2009.
Now whenever tithi changes it changes the same instance all over the world. This means if tritiya ends on 9:46 AM in India on October 7th 2009 then It will end at the same time in Europe, America, Australia everywhere as well.
Now remember when countries are behind in time from USA means we have to subtract the time zone difference from India time and When countries are ahead in time from India we have to add the time difference to the indian time. Difficult to understand? Please use pen and paper or some application like world clock to give you times in different cities of the world. This will give you idea of basic time calculations.
After doing that step check the local parameters like Sunrise, Sunset, Moonrise, Moonset. This times will vary from place to places all over the world. For more details please read http://www.mypanchang.com/Why-there-is-a-one-day-difference-in-hindu-festival-dates-in-India-and-America.php. In our case Moonrise timings are important.
Now based on that we hace calculated following tritiya ending times tithi for various places in the world.
Now let's see when Chaturthi ends in these places.
Now let's apply average moonrise timings to these cities (For actual moonrise timings please visit tp://www.mypanchang.com/panchang2009.html) which is rougly 8 to 10 PM. Now if we apply that it's clear that on October 8th 2009 during moonrise timings chaturthi tithi is not there. Hence October 8th cannot be taken as a karwa chauth. In certain parts of USA chaturthi tithi isn't visible during moonrise but it is not visible on either days and hence take the next day as per that rule we have calculated Karwa chauth dates for following places in the world:
Now remember most cities or calendar makers in india do not follow dharmashatra properly. Hence they can't explain you the detailed calculations done by pannchang siddhantis to determine real festival dates. Pundit Mahesh Shastri is a panchang siddhanti and he has put this detailed calculation of Karwa Chauth for you.
Karwa Chauth dates for 2009