you are unable to attend the farewll party for your manager at work. write a letter to him/her in your letter,say why you can't come, tell them what you valued about working for him/her , give him/her your best wishes.

Dear Madam, I am writing this letter to thanks you for the invitation of farewell party to celebrate your big achievement on 22nd of July. I would like to inform you that I can’t join the celebration. Unfortunately, I am having my practical exam date which is already listed by the university, and cannot change […]