Please explain to me in detail what happens during Monster Week and what happens during the month.
In the instance I mentioned above, Hobgoblins appeared all over the map.

Well, my knowledge about population is rather limited...:)

There are several types of population boosts:
1) +5 to a specific creature
2) Week of X creatures
3) Month of X creatures
4) Week of Plague
5) X (certain) creatures have their population doubled; so:
6) Week of Squirrels (or Groundhogs, etc.)

Regarding 1), I don't think there's any need to elaborate, it's all clear.
2) - The advantage is basically the same as in case 1), but the creatures are limited by level.
3) Creatures from level 1 to 4 are scattered across the map, and they have a "can join you" recruitment type, i.e., + this is reflected in their recruitment, and if you also have diplomacy, the chance of recruitment increases accordingly.
4) -200% growth of creatures in castles.
5) The number of units in the castle doubles (as far as I remember, this applies to Centaurs, Goblins, Riders, Dwarves).
6) There's really no need to focus on this :D

I doubt I've added anything new...