Ordinary Time in our calendar is whatever is outside Advent, Christmas, Lent & Easter. It comprises two bits: (i) day after Epiphany until Tuesday before Ash Wednesday, inclusive, (ii) Monday after Pentecost until Saturday before first Sunday of Advent, inclusive. The liturgical colour for Ordinary Time is green. There is a fair bit of complications over which day’s reading to use in the days after Epiphany and which Sundays follow Pentecost.
Ordinary Time was introduced in the Latin Rite with the 1970 reforms. Previously, the two periods were called Sundays after Epiphany and Sundays after Pentecost. Anglicans generally (this word inserted for GKC’s benefit) still follow the old naming. I am not so sure about Lutherans but I believe with the 1970 Catholic liturgical reforms filtering through to Protestant churches, some Lutheran churches (& Anglo-Catholics, of course) adopted Ordinary Time and the green vestments. Welcome to 20+ weeks of monotony after Pentecost. Of course, we get plenty of feastdays to spice things up.
Isn’t it so much easier to be Latin Rite Catholics - we just obediently follow whatever Rome tells us and so, nobody can mistake what we do

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