celebrate as in have a themed fancy dress party and knock on doors top get some sweeties, or celebrate as i knocking up some spells in a cauldron and summoning the dead ?
It's "all Hallows Eve", one explanation is that it's a Celtic celebration of the end of Summer and the start of the Dark nights. I don't know what all the pumpkin/trick or treat stuff is all about though, that's probably the American influence.
And increasingly being used in its other guise as the "night of mischief", which round here is interpreted as an excuse to go an a criminal damage rampage by certain groups of youths. Several folks cars were turned on their lid, Ho Ho Ho - but not much fun for the poor owner facing a write-off. People's houses pelted with flour and eggs and intimidatory trick and treat - more like highway robbery, car windows smashed. US style Halloween - you can keep it!