Yesterday was the start of Holy Week and various regions of Italy carry out special celebrations today on Holy Monday/Lundei Santo.
So far I have not discovered any particular rituals that take place in the Viterbo region. Castelsardo in Sardinia seems to be the most well known region that carries out a ritual today called Lunissanti. Members of the Brotherhood of the Holy Cross/ Confraternita di Santa Croce, dressed in long white hooded tunics and carrying religious artefacts, take part in processions and sing, or rather chant from dawn until dusk.
I found the following video on YouTube which will give you an idea of what goes on.