About the Castle Challenge tasks

So I missed the final reward just for ONE stupid Rare Stamp... Farewell my cute Mermaid! T_T
I think the time to complete the last day task should be 2 days and not 1 as when each new task unlocks, it's passed midnight in my Country (I'm Italian) and I don't really use the supposed 24h of game time to try completing the tasks... But while for the other tasks u can use more days (the ones when the next tasks unlocks itself), with the final one this doesn't happen and imo it sucks very much.
I think you should add another day to let the last task be completed (u know different countries means different timezones!).
Also, how often does the Castle Challenge happen?
Once a month? In the Help section there is nothing written about it...