Usually May is quite warm in Anaheim, especially during the day. Plus the humidity can start in May. However, spring this year has been exceptionally cool, cooler than winter overall. We were there last week and it was cool even during the day with the sun out (and windy). The evenings were actually cold. It was colder than when we were there in January and November. That being said, it will be unpredictable so plan to check the weather a week out and finalize your packing from there. You're going to be safe with capris and tees, but you'll definitely want sweatshirts/a light jacket for evenings and a pair of jeans.