The season is a symbolic representation of what the children of Israel did when grieved or sorrowful over their sin. They would tear their clothes and cover themselves in ashes as an outward display of mourning to move them to repentance. So Ash Wednesday is a Solemn Assembly that marks the begining of 40 days before the Ressurection. This period is symbolic of Jesus 40days in the wilderness where he fasted. So Christians fast in some form or another during this time to identify with Jesus.

It’s not really biblical or unbiblical- it’s more a day to observe your mortality and need for Christ as we prepare to celebrate Easter and what He Did for us. This podcast has a helpful explanation! https://podcasts.apple.com/us/podcast/under-the-hood/id1744827226?i=1000696508646