You will visit Hong-Kong in the end of May which is considered to be the beginning of the rainy season with the average daily temperature ranging from 24°C (75 °F) to 31°C (87 °F). The humidity level will be around 80 to 95%. Please take an umbrella and/or a raincoat.
If you are a pollen allergy sufferer, be aware many flowers bloom in May in Hong Kong: glaucous climbing bauhinia, bleeding heart vine, flame tree, lotus, etc. If needed, please prepare a mask or take some medicine.