During events, we usually get reports about the drop rates of certain rare items. Here is an article that explains how this works and a few recommendations from the team:
Drop rates is the probability of getting rare items on the foraging areas (e.g. Pond,Glade) or on several crops that is required for crafting special recipes during events.  Some events  also  feature collecting goody bags, gifts or treasures, which you need to​  open to get the special items ​necessary  for that event. 

Drop rate also apply when opening​  these gifts .

For the foraging areas, the chances of getting the item on every exploration will depend on the following:

- Quantity of Farm Hands used
There are foraging areas that have more slots for Farm Hands (e.g. Pond, Mill). The more slots you fill up, the higher the chances of you getting that item. Green means higher chances, Yellow - medium, and red has low chance.

- The Farm Hands you choose 
There are several farm hands that have special abilities that will help you gather your desired item. For example, Johnny has 70% chance of getting Chives in the Glade as compared to other Farm Hands.

During events, the game is offering a Temporary Farm Hand that has higher chances of giving the event items you need. These Helpers can be purchased using keys on the General Store and will leave once the event has ended. 

Now, on seeking that rare items on crops and animals, the probability of you receiving the item will sometimes takes multiple actions. To increase your chances:

-  You can purchase additional plots/animals
-  Hasten up the harvesting by using speed seeds
-  Ask your friends to fertilize your crops/animals to get twice the number of rare items