To get the quest for this, you need to have completed the first requirement for Snake in the basement of your Church (which also gets you the 'Summon a chicken' achievement). Snake will then send you to the Astrologer to get a specific book.
The Astrologer appears by the lighthouse on the far right of the map on the day which has the little blue crescent moon symbol.
Make sure to take 5 Maggots with you on the way as it will save you time when you get there. You can get Maggots by creating a compost heap in your garden and generating compost with dead plant matter you get when you harvest crops.
Once you reach the Astrologer and ask him about the book, he will tell you he gave it to the Fisherman a few steps nearby standing directly next to the lighthouse.
Head over to the Fisherman and ask him about the book. He will then tell you he'll give it to you for some Maggots.
If you brought Maggots with you, it's a straight swap and you can head back to give the book to Snake when he next appears in the basement of the church. As soon as you hand it over, the achievement should pop.