Pascha is the anchor
The Orthodox year turns around Pascha (Easter), not Christmas. The date of Pascha decides the dates of Lent, Pentecost, and half of everything else you see on the calendar.
Two overlapping cycles
A movable Paschal cycle floats each year with the date of Pascha. A fixed calendar cycle (like feasts tied to December 25) stays in place. Every day is the intersection of both, which is why the same civil date can feel very different from one year to the next.
Old Style / New Style
Alongside the civil date you may see a Julian ("Old Style") date. Many Orthodox jurisdictions still reckon fixed feasts by the old Julian calendar, which currently runs 13 days behind. This is a living tradition, not a bug.
Fasting as rhythm, not diet
Orthodox fasting is a shared rhythm: usually no meat, dairy, fish, oil, or wine on the strictest days, with wine and oil allowed on lighter days. There are four fasting seasons — Great Lent, the Apostles' Fast, the Dormition Fast, and the Nativity Fast — plus Wednesdays and Fridays through most of the year.
Every day is someone's day
The calendar lists saints and events remembered on a given date, not because the Church runs out of names, but because Orthodoxy takes seriously the "great cloud of witnesses" of Hebrews 12:1. Saints are seen as living intercessors, not distant historical figures.
This is the Slavic tradition
The calendar shipped with this app follows the OCA / Russian-tradition practice. Greek, Antiochian, and other Orthodox jurisdictions overlap almost entirely, but you may see small differences in feast rank, fasting dispensations, or particular commemorations.