April and May are the standout months. This is when yellowfin tuna run largest and the sea tends to be at its calmest for the whole year, which makes for an easier ride out and back. Longfin tuna and snoek are usually around too, so it’s rarely a wasted trip even if the yellowfin aren’t biting that day.
If April and May don’t fit your schedule, aim for October to December instead. The water is warmer and the weather more forgiving, though the tuna you catch will typically run smaller on average than the autumn fish.