During some events you are able to collect special resources for which you can get a reward. These events are the easter event during which you can collect Easter eggs , the Halloween event during which you can collect Halloween pumpkins  and the Valentine's event during which you can collect Valentine's hearts.

During these events occasionally an Easter egg / pumpkin / heart will appear next to the name of one of your missions in the mission screen. Important: You have to click on the egg, pumpkin or heart symbol in order to collect them.