the Inca calendar was important for religious reasons
First of all there is no such thing as the American calendar. The calendar used throughout the world for business and commerce is the Gregorian calendar. All calendars go through various stages of evolution as refinements were made over time. As nations conquer nations and get assimilated then the dominant and usually the most accurate become the one used for commerce. Though others may be kept for religious and other reasons. There was never an overall Gallic calendar as the Gallic people were widespread and had various beliefs. Mostly they would have relied on the calandar the Romans used.
reasons why children development is not following expected patterns
No one person invented the calendar. Many cultures all over the world developed them for various reasons. Some were purely for rituals others reflected the seasons or phases of the moon etc. As people became more sophisticated better and better and more accurate calendars were introduced.
Ashura is a significant day in the Islamic calendar observed by Muslims for different reasons, depending on the branch of Islam. For Sunni Muslims, it marks the day when Prophet Musa (Moses) and the Israelites were saved from Pharaoh's tyranny. For Shia Muslims, it commemorates the martyrdom of Imam Hussein, the grandson of the Prophet Muhammad, at the Battle of Karbala in 680 AD.
Yes, if necessary for birth control reasons.
Muslims do not eat halal for scientific reasons; they do it because God commanded that they eat this way.
It depends on what your Following Reasons are.
There are various reasons why Muslims need to get ready again for prayer. All the reasons are to appear right before Allah during prayer. Getting ready will entail washing the face and having the right attire for prayers.
For the same reasons all other faiths do - to pray and rest.
lack of knowledge
Their traditional calendar was aligned to the agricultural seasons. After Alexander the great took over Egypt, his Macedonian successors introduced a new calendar, modified later by the Romans as the Julian calendar. A modified Egyptian calendar has remained in use by the Coptic church and peasantry.