You can see the sights of the city in a day, easily, so you may not want to plan more than a night in the city. However, there are two big festivals held in Phitsanulok every year. A big temple fair is held in the grounds of Wat Phra Sri Rattana Mahathat every January, while the river running through town hosts very popular long boat races every October. A longer stay at these tmes may be in order.