Hallowe'en?

Karin is always happy to attend a party, but what on earth is this 'Hallowe'en' thing? Is one supposed to dress up? Is it like a masquerade? Whatever is a poor Swede to do?
Thankfully she knows someone that knows everything that one needs to know about parties. And hopefully Mr. Attford won't mind answearing some questions, so she sends a note, by raven, to her friend.

Dear Nathan!

I need your help!
What is Hallowe'en, and how does one prepare for a Hallowe'en party?

Sincerly
Karin.