The story
Estrenar is Spanish for using or wearing something for the first time. The verb comes from estrena, an old word for a gift, and through Latin strena it is related to French étrennes, the presents given around New Year. To estrenar something is a little like unwrapping it.
The word covers a lot of ground. New shoes are estrenados, and so is a car just off the forecourt or a flat nobody has lived in. A film or a play has its estreno, its premiere, and someone doing something for the first time se estrena. Catalan has the same verb and even uses it for a shop's first sale of the day.
What makes it hard to put into English is that English picks a different word for each case. You break in new shoes but christen a new kettle. The feeling underneath, that the thing is new and you are the first to use it, has no verb of its own. Christen comes closest, and the joke in it, borrowing a church word for a saucepan, shows how much English wants one.
A scene: the first day of term in Seville. An eight-year-old waits at the school gate in shiny new trainers. His friend looks down and asks, '¿Estrenando zapatillas?' The boy nods, and spends the rest of the morning keeping his feet carefully out of the puddles.