When they mean "happy/delightful/pleased",no big difference. Following are differences off the top of my head ;P : “愉快”is a bit more formal 1.can be used to describe a thing, eg "愉快的旅行" 2.as a wish:"周末愉快！""合作愉快" “快乐”is both a none, an adjective, and a verb. eg1"你快乐吗？”(as a verb) 2.as a wish “生日快乐”"圣诞快乐""节日快乐" In other contexts they can supplant each other i think.....feel free to ask futher questions

actually ，there isn‘t difference between both

快乐：感到高兴或满意 愉快：心情爽快舒畅 口语中一般用快乐！