What a lovely, mysterious story - with a heart-warming ending. You've gone for a different angle on the Christmas theme and it works very well; having the pagan festival as the focus allows you to avoid most of the standard Christmas tropes while still conjuring up an appropriately wintry and magical feel. Puzzle-wise the difficulty level is quite easy, which feels right for this jam, and the code-breaking element was a pleasant diversion (nostalgic of the kind of thing that used to be included in every Christmas annual). Having the archaeologist character on-hand to help with the puzzles is an effective way of integrating assistance into the game without reverting to a blunt 'help' command, and provides a satisfying denouement at the end. The low-res graphics (reminiscent of 'Over Here' from the last jam) are perfect for this, I think.
There aren't many bugs in it: I spotted a miscapitalisation and a missing word or two in your prose, and 'put x on altar' produced the standard 'the altar is not a container' response that you might want to override. Also, I'd suggest some sort of acknowledgement when you cross the ice. I confess that I found the means to cross it and did so without even realising that I had done it - it took me a while to realise that the location I was then in was on the island.
Overall this is a great effort and I really enjoyed playing it.