I'm Hindu and I've noticed that in some places, Sravan month ends a few weeks earlier (eg. in Nepal it's already over), than for other places (e.g my town). Why does this happen?
Sravan month is just one of the examples; these happen during most of the festivals.
I've tried to ask some religious people but have not been satisfied by the answers.
What's the need for so many different calendars? Why can't we have one that will make our religion more powerful and less doubtful? This QA addresses two calenders only, but I've noticed there are more than two.
Normal calendar made by religious experts.
Vaishnava calendar (varying from 1 city to another).
North Indian calendar (again made by some other religious experts).
For some other South Indian States like Tamil Nadu (made by their experts).
There might be many others about which I'm not aware.
The calendars will differ. Because the sun is rising at different timings in different regions. Calendars are actually a part of culture than based on a religion. Those are based on mainly the Sun. Most of the Hindu rituals are related to the nature and weather conditions. So according to the regions, these factors differs. Since Sanathana Dharma is the culture of India, the different calendars in Hinduism depend on the weather and nature conditions of India.
Most calendars will have mentions of the start of the agricultural season. Just consider the monsoon, the starting and duration of monsoon varies in different states, according to that there might be differences in calendars in South India and North India. This culture is very much related to agriculture, and it is a wide spread culture in a large country. There can be some variations in the same culture which will be most probably based on the nature of that region. So the festivals of specific regions also depend on this. Most of the festivals in India will be in the reaping season of the agriculture, so those festivals will vary according to states.
For example in Kerala, there is Malayalam calendar starting at the month of 'Chingam' and it is the reaping season. So one of the most celebrated festival in Kerala, 'Onam' is in the month of Chingam. So from states to states festivals may differ. There will be common festivals, but most of these festivals and rituals depend on the weather and nature. Even if there are some common festivals, each of them will have it's unique variations.
Each Smrithi is followed in a region for a particular period according to it's nature and weather conditions. Smrithies are allowed to edit accordingly whenever needed. So the experts in particular region could have edited them according to the conditions. This is the way our ancient rishis and experts defined it and it should not be changed unless there is variations in nature conditions.
Calendars are often designed with the solstices, equinoxes, and alignment of heavenly bodies in mind; in conjunction with the earth's wobble. The problem is that these differ depending on where you live, especially the heavenly bodies.
Although, as already mentioned, they are more cultural then religious.
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