Explain to me that it is opposite day. On opposite day, everything you say is reversed. "No" means "yes", "is" means "isn't", and so on. Because you are trying to explain to me it is opposite day on opposite day, the task isn't simple. For example:
If you tell me it is opposite day, you will be saying that it isn't opposite day, and therefore won't convince me.
If you tell me it is not opposite day, I will simply believe and agree that it is not opposite day.
What can you tell me to convince me, without a doubt, that it is opposite day?
Edit: For those asking, the sentence's meaning is reversed. So "I am going to tell a lie" turns into "I am not going to tell a lie".
Edit #2: For those claiming the riddle is too broad, opposite day has been revised to changing meaning. There is a specific task and specific parameters.