Paganism was a term for pre-Christian polytheistic religions.
The Second Doctor, Ben Jackson and Polly Wright visited the White Rabbit in the 17th century in time for Christmas, but were quickly arrested for mentioning the pagan holiday. (PROSE: The Feast) Indeed, according to Rassilon, Christmas was a pagan rite to banish the cold and the dark. (TV: The End of Time)
Mistletoe was brought into homes in winter with other evergreens as a pagan offering to pray for the spring to come. (PROSE: The Doctor's Cross Word)
The Nine Travellers were known to have been part of pagan rituals. (TV: The Stones of Blood)
The Seventh Doctor called August bank holiday a "silly pagan festival". (PROSE: Lungbarrow)
While in Sweden in December 1070, the Eighth Doctor and Izzy Sinclair were captured by a band of pagan hunters. (PROSE: Illumination)