How often are land sales?

[Nutster]

I have noticed a couple of times, over the last few weeks, that the land to expand my airport or city goes on special at 50% of the green cash, and sticks around for a day or so each time. I have managed to take advantage of that offer once so far, and clearly would like to again. So, how often do those land sales happen?

 

[Fencat]

Every second weekend for the moment atleast so should be again next weekend