During some events you are able to collect special resources for for an award. These are:
IconResource NameEvent
SnowmanWinter Event
Valentine's HeartValentine's Event
Easter eggEaster EggEaster Event
SunflowerSummer Event
PumpkinPumpkinHalloween Event
SantaChristmas Event

During these events occasionally the icon will appear next to the name of one of your missions in the mission screen. Important: You have to click on the icon in order to collect them.