A casual barbecue is all about the vibe, not the stress of a massive bill hitting your account at once. Getting the timing right is how you keep it relaxed.
You should aim to buy your bulk meats and drinks a few days before guests arrive. This gives you time to organize the fridge and avoid any last-minute grocery store panics.
Remember that the 0% intro APR on purchases is a temporary window, not free money. If you have not paid off the full $350 by the end of the 15 months, the remaining balance will start accruing interest at the standard rate.