WHERE HE WAS WORSHIPPED
Gaul, Britain and Hispania, though mostly on the continent.
HOW HE WAS WORSHIPPED
Taranis is mentioned by the poet Lucan in and is associated with various gods of other lands, like Thor, who is likewise a god of thunder. He is also associated with the wheel, i.e. the wheel of time. Apparently he controlled this by way of ritual matings with Diur, or the oak tree.
He's a powerful god, and apparently his followers went to equally intense lengths. Lucan makes accounts of people being burned alive in honor of Taranis, and cut to pieces, as well as possibly the death in the 4th century BC of one man at least.
These ideas are contested, however, by other anthropologists and archaeologists, who insist that since Taranis is really only the embodiment of thunder - his name literally means "thunder" - then this would have made no sense; he was more of a generic nature god. Also, his name has only been found on seven surviving altars, as opposed to Lucan's insistence there was a wide cult devoted to this god.
There is still a discussion about whether or not Taranis was as important to the Celts as Lucan suggests, but meanwhile theories still appear. Newer ones include the concept that worship of Taranis might have led to the famous burnings of the Wicker Man at solstices in modern times.
TARANIS IN MYTHOLOGY
Other than Lucan's accounts, which may or may not be correct ones, there is little written information regarding this god. The few things we know about the Celts and Taranis are these: he was apparently part of a triad including Esus and Teutatis, both powerful gods in their own right. He was thought of as the god of thunder, and possibly worshipped at such times as there were storms, though no songs or poetry about him in this state survive.
HOW HE IS WORSHIPPED TODAY
Some people light a candle and sing to Taranis when he makes himself known in the sky, while others will cast protections on themselves and their objects using some of his magic. This can be done easily by carrying a lightning-struck piece of an oak tree. Mistletoe is another plant sacred to this god, if it can be found.
Otherwise, his sacred day is apparently the 7th of July, at which point the Caledonii celebrate him for three days (6th, 7th, and 8th) with song and fire, commencing in the consecration of new Draoi.