Hello, i'm glad tha you're enjoying the game :)
"path to the swamp" area unlocks after about 20-30 in-game days (not real world days). It is located in front of Black's Barn : from Nishy's home location, go towards TC, then to the right, then to the left, towards big rocks region (there's a sign on the right of it's entrance showing directions).
P.S.: Only this and the Mofu Lake area (located near the blacksmith's region) are unlock after passing some in-game days. There are a few extra areas (Beach, Winter) for the special events, which happen once every 17 days.
Cheers,
~NK