If a product is marked as seasonal, the forecast will give more weight to sales 12 months ago instead of the latest trends. This is a great way to account for holiday spikes!
Seasonality can be enabled account-wide at Account > Settings > Forecast.
If a subset of your products are seasonal, it would be better to leave the default (non-seasonal) forecast in place. Use filters in the Replenishment report to find the seasonal variants (e.g. filter by category or vendor). Then select the items.
Use Bulk Actions > Enable seasonality.
You can also enable/disable seasonality when editing a specific product in its Forecast settings tab.