Why do many Christians insist on using Friday as the starting point of answering "when was The birth of Jesus", BCE6-1

When the year of the birth and crucifixion of Jesus is used in dating events and prophecy in the Old Testament, most often Friday is used as the starting point as "it is the one known date", so that is "Passover, Nisan 14 is Friday"! The claim is "Jesus rose again on a Sunday and thus He was crucified on Friday (Luke 13:32). There are only a few Fridays possible: April 7th AD 30, April 3rd AD 33, or April 23rd AD 34. (Ex: April 23 AD34 was Friday, Nisan 15: however it is possible there was cloud cover at the end of the previous month which would have made it Friday 14th Nisan.) This starting point is already flawed. Was Jesus crucified on a Friday or on Nisan 14 and ressurected on the 18th, the 1st day of the week?? The Lunar calendar can never have Nisan 14 falling on a Friday in consecutive years, (There are 354 days in a lunar cycle of 12 months, 384 days in 13 months) so why is that ignored when making the calculations for arriving at the dating of the Old Testament events. Therefore, in arriving at the dating on Old Testament cannon you cannot just change an event, "4 BC should be 5 BC because Jesus was crucified on Friday.