Examine the concept of seasonality in business, focusing on the impact of events such as holidays on sales. Discuss the significance of understanding and forecasting seasonal variations for retailers. In the context of a scenario where an electronics retailer experiences a $25\%$ increase in sales during the holiday season and has average monthly sales of $$500,000,$ explain the calculation of the expected increase in sales. Analyze how retailers can leverage this knowledge for effective inventory management, marketing strategies, and overall business planning during peak seasons.