how we can buy tickets for gulangyu island? Is it we have to purchase ticket few days before?

Oh, if you have Chinese friends, you can ask help for them to book tickets online in advance. Otherwise, you can just buy the tickets when you arriving at the scenic area. Generally speaking, there will have enough tickets for you. Don't worry.
According to the weather forecast, the temperature will be 60 F to 75 F tomorrow. So you can prepare light jackets, long sleeves and pants. Have fun!