RORY: See you for some tree planting over at the Arbor Day Festival, buddy.
Arbor Day is an international secular day of observance in which groups and individuals are encouraged to plant trees. It is usually held in the spring. In the US, Arbor Day was founded in 1872 by J. Sterling Morton in Nebraska City. By the 1920s, every state in the US had laws stipulating that Arbor Day had to be observed.
In the US, National Arbor Day is celebrated on the last Friday in April, so it is two weeks away from this episode, and Rory is anticipating another event she might be able to get Jess to attend. We never see an Arbor Day celebration in Stars Hollow, but of course they hold one every year. By the time Arbor Day arrives, Jess has left town.
The Annotated Gilmore Girls 