Hop-tu-Naa is a Celtic festival celebrated in the Isle of Man on the 31st October; elsewhere known as Hallowe'en.
For Hop-tu-Naa children dress up as scary beings and go from house to house with the hope of being given sweets or money, as elsewhere. However the children carry turnips rather than pumpkins and sing an Anglicized version of Jinnie the Witch. The changeover from turnips to pumpkins has also happened in Scotland, where the similar practice is called "guising".
In older times children would have also brought the stumps of turnips with them and batter the doors of those who refused to give them any money! (An ancient form of trick or treat, however this practice appears to have died out.)
The term is Manx Gaelic in origin, but its meaning is actually unknown. It may be cognate with Hogmanay, which is the Scottish New Year; Hallowe'en being the original Celtic New Year.