ABIB 15 HIGH DAY ERROR

Why do some people observe Passover on Abib-Nissan 15 instead of on Nissan 14?

Why did God include some scriptures that seem to establish Abib 15 and 21 as the holydays?

Celebrating ABIB 15 as the HIGH DAY is a serious doctrinal ERROR that cuts one off from being a true disciple of Jesus Christ who keeps the Passover on Abib 14 as Holyday 1.

See:
https://pentecostnation.freeforums.net/thread/210/abib-15-high-day-error