Dear X,
Thank you for your invitation,I appreciate it very much.Since I have other plan (I need to go somewhere,.,罗列原因) for the weekend (如果你们约在周末),I'm afraid that I won't be able to join you.Wish you have a great time in XX（游玩地点）!
Best regards,
你的大名
Dear X,
Thank you for your invitation,I appreciate it very much.Since I have other plan (I need to go somewhere,.,罗列原因) for the weekend (如果你们约在周末),I'm afraid that I won't be able to join you.Wish you have a great time in XX（游玩地点）!
Best regards,
你的大名