Does a fire in a fireplace burn better when the outdoor air is calm or windy?

The fire in a fireplace burns well when it is drawing well, which means it is getting a steady flow of fresh air (& oxygen) to the fire and exhaust/smoke is flowing steadily away. An open fireplace in a building needs quite a large volume air to be supplied to it (either from a draughty or well ventilated room, or through a vent), and a large exhaust through the chimney.

The wind around the building can affect the drawing of the chimney by creating high and low pressure areas. If the chimney top is in a lower pressure area than the room with the fireplace, the air will draw steadily through the fireplace and chimney, because air in a high pressure zone will want to flow to a lower pressure zone. If the chimney top is in a higher pressure area than the room, the flow will tend into the room, bringing the smoke with it.

Wind direction and speed can affect this, as can the position of the chimney opening and the ventilation of the fireplace room. A well designed fireplace should draw well in all wind strengths and directions, but many do not.