In Greece Carnival is also known as the Apokries (Greek: , "saying goodbye to meat"), or the season of the "Opening of the Triodion", so named after the liturgical book used by the church from then until Holy Week. One of the season's high points is Tsiknopempti, when celebrants enjoy roast beef dinners; the ritual is repeated the following Sunday. The following week, the last before Lent, is called Tyrine (Greek: , "cheese [week]") because meat is forbidden, although dairy products are not. Lent begins on "Clean Monday", the day after "Cheese Sunday". Throughout the Carnival season, people disguise themselves as maskarades ("masqueraders") and engage in pranks and revelry.
Try to answer this question if possible (otherwise reply "unanswerable"): What does the Opening of the Triodion derive from?
the liturgical book used by the church