A:
A flavor of the month ("flavour" is the British spelling) is literally a special flavor of ice cream that is only available in a particular ice cream parlor for one month. As a metaphor, it can mean anything that will be gone or forgotten in a month.
Herding cats is trying to coordinate a group of people who refuse to cooperate or listen to you. Cats are not very social animals and they don't like to be trained, so herding them (getting a large group of them to all move in the same direction in a big open area) would be much more difficult that doing the same thing with dogs, cows, or sheep.