Ordo is the quietest room in the house: today in the Church, at a glance. A link to the day's Gospel reading, and — on a solemnity, feast, or memorial — the name of the saint the Church remembers, with a short life you can read without leaving the page.
Most of those lives are drawn from Alban Butler's Lives of the Saints (1894), now in the public domain. Butler wrote before many modern canonizations, though, so where he is silent the life is taken instead from the Holy See's own words at Vatican.va — a papal homily, catechesis, or biography — and, failing that, retold from Wikipedia in the Church's devotional voice. Every excerpt names its source and links back to it.
The calendar itself is the General Roman Calendar; the Gospel text lives at USCCB, linked out rather than reproduced.
The Church keeps time by the life of Christ. The year turns through seasons — Advent and Christmas, Lent and Easter, with Ordinary Time between — each with its own colour and mood, so that over twelve months the whole mystery of salvation is lived through rather than merely recalled. Every Sunday is a little Easter; and threaded through the seasons are the days that remember the saints, the friends of God held up as proof that holiness is possible. Not all such days weigh the same, and the calendar ranks them.
Days with no saint of rank are simply weekdays of the season — feriae — and on those Ordo shows only the Gospel.