If an event is semi-formal but balances most on the casual then chinos can be your attire. Here’s how you match chinos in a semi-formal event - chinos i.e. a white and blue oxford shirt plus dress pants or a trouser plus blazer. You can go with the unstructured blazer also no need to put on a formal blazer. If you want then you can avoid a tie which is fine by the meaning of the event as semi-formal. Here everything is formal but the absence of the tie will make you feel a bit more relaxed than a formal event.