We might say this to show exasperation or shock that the person doesn't realize the importance of that specific day. In your example the importance of that day is that there is a famous sporting event happening. Question: Why are you so excited? Answer: It's ONLY the day when the world's biggest sporting event is kicking off! Hope that helps. I believe it is an adjective in this sentence.

It is a sarcastic comment. Only the day of the biggest event of the year or picture referring to your wedding day as just "a day" Or the day of your first child's birth as just a day. Of course it is a very important day, but the writer saying its "only the day is being sarcastic in their meaning. Kicking off means the start of the event of the beginning of the day. In football, the kickoff is the start of the game. So kicking off is the beginning.