Hello Allá. In mid-May, the weather is usually milder, although you may encounter several rainy days depending on the area of ​​Spain you're traveling to. If you plan to use public hostels, you should bring a sleeping bag. Many of these hostels also have blankets that can be used free of charge most of the time, but they don't have sheets or pillowcases. However, if you're not using public hostels and are choosing private accommodations that you've pre-booked, most of them do have sheets and blankets, so you don't need a sleeping bag. You should check with each accommodation in advance to find out what type of service they'll provide. Best regards.