When most of your products have more than 12 months sales history, you can use the seasonality to estimate the spike. Given a seasonal product, the forecasting algorithm puts more weight on sales 12 months ago and therefore accounts for the increase in sales (if any) of each particular product.
To enable seasonal forecasting, select products in the Replenishment report.
Click Bulk Actions -> Enable seasonality.
For other methods of editing your forecast, see our article about forecasting for the holiday season.