According to Lionet:
"the moon will be full around the 12th-14th each month (the 13th of each month being the prime time for the events on Grimvale). We introduced these "moon phases" to be a constant phenomenon around the 13th of each month during the whole year." (source)
This means depending on the time of the month you are visiting Grimvale, you either encounter fairly harmless creatures such as Badgers and Bandits, or a lot stronger creatures such as Werebadgers, Werebear and Wereboars. Also, you will either encounter Purple Nightshade or Blooming Purple Nightshade.
|Cornell||Ferryman||✗||North of Stonehome and south on Grimvale, approximately here.|
|Gladys||Druid||✓||Southeast on Grimvale, in her little house.|
|Maeryn||Guard||✗||South-west on Grimvale, in a small house.|
|Owin||Hunter||✗||On Grimvale, northernmost hut.|
Surface: Not Moon time