Most of the tourist come to visit Ijen Crater at the dry season April until September (in the same time with summer in Europe). Commonly knows that temperature differences could be significant between day and night. Hot in the day, and very cold in the night. You must prepare all your needs. Do not say if you not need jacket, or something warm in Ijen. Even the summer is very hot, but you can feel freezing during your tour here. Do not forget to bring hand gloves, head scarf, or blanket. You will need those goods.
If you come to visit Ijen at rain season, do not worry, most of the day during a year the top of Ijen is often rain. You can enjoy the scenery of Ijen, therefore the rain falling in the location around it. For your preparation, bringing the raincoat and bag waterproof could help you so many.