Smart casual means many different things to many different people, not to mention the kind of event or destination you’re heading to. Smart casual at a graduation will likely mean something very different to smart casual on date night.
So when dressing for this ambiguous dress code, it’s important to consider where you’re heading and who you’re heading there with. This will inform just how smart, or just how casual you go. For example, if you’re heading to a graduation, a blazer is a failsafe part of that outfit. Graduations tend to be fairly rigid and formal, and those participating will be wearing gowns and caps, so you certainly wouldn’t be overdressed. Wearing clothes on the smarter end of the spectrum also acts as a sign of respect for those graduating – you’re showing appreciation for how significant the event is.
For a date however, wearing a blazer might be a bit much, depending on where you’re going. In order to not feel overdressed, you might prefer to go fairly smart on the bottom half, with tailored trousers and desert boots, but keep things more laid back up top, with an untucked Oxford shirt and bomber jacket.