The optimal time to purchase property in Koh Samui for obtaining a higher rental yield is during the low season, which typically runs from May to October. This is due to the decrease in tourism during this time, resulting in lower rental prices and higher bargaining power for buyers. Additionally, investing in properties located near popular tourist areas and attractions can also lead to a higher rental yield. Using these factors in your decision-making process can ultimately result in a more successful and profitable investment strategy.