Someone wrote to me last week asking about Ash Wednesday and our observance of it, despite no Scriptural reference to this particular day and the rite of imposing ashes. Here's my answer:
Regarding Ash Wednesday, almost nothing in the order and structure of Christian worship is explicitly specified in Scripture. Christians did seem to worship on the "Lord's day," i.e. Sunday, but there was certainly no provision for festivals like Easter, Christmas, or anything else. All of these things were developed as ways for the churches to order their practices of worship. The liturgical season of Lent is very old, going back at least to the 3rd century, and the use of ashes in connection with mourning and penitence goes back well before the time of Christ. Ash Wednesday emerged late in the first millennium in the Latin churches as a way to mark the beginning of Lent, which had been a season of fasting and penitence for several centuries by then (Ash Wednesday is not practiced in the Eastern churches, as far as I know). The Lutheran churches retained the seasons of the church year, including Lent, and continued to celebrate Ash Wednesday, though in many places the imposition of ashes itself was suppressed (along with the palm procession on Palm Sunday!). In recent decades, Lutherans have recovered some of these practices which are neither commanded nor forbidden by Scripture but which enrich and deepen the experience of worship.
Luther wanted to simplify church calendars but he did not attack the seasons or their major observances. The critical distinction for Lutheranism wasn't between "Scriptural" and "man-made" rules necessarily, but between what is specifically commanded or forbidden and what is optional. Ash Wednesday falls into the "optional" bucket--the Eastern churches never practiced it--so it could be dispensed with for some good reason, but it does not in any way contradict the precepts laid out for worship in the Scriptures.
Some of our Reformed and Baptist brethren go farther than Luther, insisting that only Scripture be used as a source for rules and orders of worship. The problem with that view is that no such rule or order exists in Scripture, and the church did worship just fine for a long time without needing to refer to Scripture for every single practice and custom. Much of what happens in worship is in fact older than the written New Testament, and it's probable that some New Testament passages originated as worship texts or songs.
If you're interested in a very early, fairly extensive picture of early Christian church life, check out the Didache. It's roughly contemporary with the New Testament and has a lot of information about how some churches may have been worshiping and organizing themselves before there was any agreed-upon list of books called "Scripture."
God's Work. Our hands.