It is swung at the special Friday evening mass, and occasionally at other masses too.
I witnessed the occasion at Friday evening mass last week. It takes place at the end of the mass. The censor is lowered, filled with burning charcoal, then with eight men pulling on ropes, it swings into the transepts, almost to the ceiling.
It is quite dramatic when first seen, accompanied by loud organ music.
Each day at midday a special mass for pilgrims.
The clergy in attendance were robed in purple not green. Those pulling the ropes robed in claret.
Friday and Saturday, the censer was stopped. On the Sunday it was allowed to swing and slowly slowly come to a halt.
The description on Wikipedia, like most things on Wikipedia, false. No way was 40kg of charcoal and incense shovelled into the censer. Nor did the swinging censer fill the cathedral with smoke or produce large volumes of smoke. Botafumeiro is normally found suspended at the end of the rope, not kept in the cathedral library.
The Botafumeiro is suspended from a pulley mechanism in the dome in the centre of the cathedral, the entire mechanism flexes as it is swung.