3 months ago - Shurenai - Direct link
Blood Moon Frequency = Minimum forced frequency of blood moons
Blood Moon Range = Variance from the above forced frequency

So if you have Freq 7 Range 0, You will have a horde every 7th day 100% of the time.
If you have Freq 7 Range 14, You will at minimum have a blood moon after 7 days, but potentially after as many as 21 days.
If you have a Freq 1 Range 21, Then you potentially could have a horde every day, Or it could take as many as 21 days; Statistically averaging somewhere around the middle.

Functionally, Option 2 essentially meets your request; Although you further define a minimum of 1 off day, so a frequency of 2 would be your goal, which is easily achieved with a small modlet that adds 2 to the list of options.