In 1.5 Edition Emily visits Sandy in the desert for her Sandy's birthday on Fall 15, closing the store (More research needed).
However, the Oasis is locked on Winter 25. On Winter 25, despite the Feast of the Winter Star, Pam will still drive the bus as on her regular schedule, allowing the player access to the desert.This means that you can only travel to the desert by using a Warp Totem: Desert or Desert Obelisk. If Pam is at her doctor's appointment or visiting Ginger Island the bus will not run.