I think e'en is a Scots word (abbreviation of even as LRD points out). In Scots, I think the apostrophe would normally be there, but the word has been anglicised in Halloween. I am fond of the Scots punctuation with the apostrophe. 
It's still Hallowe'en where I am. I discovered today that there is an actual word for fear of Hallowe'en. It is Samhainophobia. I love that.