There are many social rules and etiquette that pretty much go "unspoken." They are the rules that you do not have to really "say" but everyone knows and follows throughout their lives. One of them is about drinking and alcohol consumption before attending an event—especially a family event.
Many people know that it is in poor taste to show up to a family event, especially a kid's event, wasted or drunk. It's not appropriate, and it's also offensive to other people who are attending. Doing so can cause a rift or a divide between you and your family, one that may be hard to fix.