Holly week is defined to be the week leading up to Christmas Day.
In this week it is traditional to cram in as much ringing as possible - fully-unmuffled ringing is preferred, although some religious types have decreed that half-muffled ringing is more suitable as "we are mourning the fact that an event that has already happened has not occurred yet..." 
Special methods such as Holly Surprise Maximus should be rung to peals. If you are unable to achieve these methods then a close substitute is having a member of the band called Holly.
Holly can be wrapped around the sally for an authentic holly week experience. Legend has it that this was the way original ringers were punished for muffling/unmuffling the bells during this most sacred of weeks.
- See Ringing in Lent for further examples of this logic.