In the NYE fireworks Meri, and her cat Snoozi, see an invitation to a party from the fairies.
The next day Meri and Snoozi go to the oak tree at the invited time and, sure enough, there is a tizzy of fairies waiting for them to join in the celebrations for the full moon.
Each month, when there is to be a full moon, a new invitation is sent to Meri in some magical way and she and Snoozi join the magic folk to have some fun.