Are observances like Ash Wednesday & Lent biblical? May a Christian participate in such things?
The observance of Lent is not a biblical practice, per se. The Bible never commands it nor does it prohibit it.
Lenten practices are rooted in the traditions of Catholicism and were later carried forward by Protestant denominations coming out of the Reformation. Under Catholic doctrine, Lenten sacrifices (e.g., fasting, abstaining from certain foods, etc.) are a way to "share in Christ's suffering" so Catholics might also share in His glory, according to Catholic misreading of Romans 8:17. In other words, Catholicism teaches their followers that to be glorified with Christ (i.e., to be saved) requires that we first endure deprivation and suffering like Christ did on our behalf, and Lenten observances further that goal.
Obviously, such teaching is heresy. First, the Bible clearly teaches that no one is justified (i.e., saved) by works or suffering but only by faith in Christ, Who suffered on our behalf. Observances like Lent cannot reconcile us to God, since a work of sinful flesh cannot erase our debt before God. Only faith in the shed blood of the sinless Christ for our sins can reconcile us to God. Therefore, anyone who practices Lent in the expectation of earning or maintaining their salvation has fallen from grace or was never saved in the first place.
Secondly, Catholicism's justification for Lent taken from Romans 8:17 is a blatant twisting of the scriptures. When understood in its context, Paul's teaching does not support the Catholic view. Instead, Paul instructs believers not to fear for their eternal security when they experiencing suffering on earth. Rather, Paul reassures us that our suffering on earth can be compared to Christ's suffering on the cross: it will be temporary and ultimately leads to glory with Christ. Catholics teach a false Gospel of salvation through works, so they misinterpret passages like Romans 8:17 in order to support their views.
Therefore, a Christian should approach Lenten observances with caution and discernment. While observing a Lenten season is not necessarily sinful, the theology associated with the practice is potentially confusing or detrimental to new or untaught believers. The various arguments for Christians observing Lenten practices may imply the necessity of such sacrifices for pleasing Christ or maintaining our fellowship with Him, but the Bible says Jesus made the once-for-all sacrifice for sins, and therefore no more sacrifices are required (Hebrews 10:10).
Instead of pursuing a Lenten season of self-sacrifice, we recommend Christians engage in a regular practice of fasting, prayer and exercising self-control in all things in keeping with Paul's teaching in Romans 12:
Rather than dedicate a certain period of the year to meaningless sacrifices, we are better to devote our life to pleasing Christ by making our bodies a living sacrifice of praise to Him.