Summarize the following:
Upon being invited, honestly and clearly state why you don't think attending would be a good idea. Your reasons can span quite a few possibilities, including running into a recent ex, not having enough knowledge of the event in question, or even that you are too tired and need a night to yourself. Some people will be offended by you honestly expressing a desire to stay home. If this is something you can handle--great. If not, you may need to use one of the aforementioned techniques. Once you've expressed your concerns, reiterate why your presence is not needed. Once you've expressed your concerns, the host might try to convince you to change your mind, or have counter-arguments as to why you should come. Explain why your presence is a bad idea, citing example scenarios. You could say, "My ex is going to be there, and I'm not ready to see her yet." You could also say, "I am exhausted, and that is my only day off." When you turn people down, they are often flabbergasted and want reasons for your reaction. While you do not have to answer any questions they pose, do try to be gracious in your responses and provide any answers you feel comfortable giving. When you answer questions, do so with respect. If you are not comfortable with the types of parties the host throws, try to phrase your reason carefully, such as, "I'm not much of a drinker, so I always feel awkward and out of place." At the close of the conversation, the person inviting you may offer a last-ditch attempt to get you to come. Many people end up caving in at this stage out of guilt and accept the invite, but be (kindly) firm in your "no." It might hurt your host's feelings, but they will likely appreciate your honesty. Do not give in, then change your mind and cancel later. If you do not want to go, say so directly. If you give in, stick to your commitment.
State your concerns about going. Explain why you don't think going is a good idea. Answer any questions the host might have. Be kind, but firm in your decision.