Special offers (frequency)

Hi all! Im kinda new here, i started THIS game a month ago. I would like to know how often the game offers special deal, when i can buy gems for better prise. I know That in every month on the 2. Week {thursday} there is an event, for which the game will surely offer Some special deal. But the other offers meanwhile? Thx your answeres in advance, dont be shy to correct me if Im mistaken about the above mentioned.