You can find the corresponding options in the settings of the Taobao merchant backend to set the delivery time during the Spring Festival. The specific operation steps may vary according to the version and update of the Taobao merchant backend. Generally speaking, you can follow the steps below to set it up:

1. Log in to the Taobao seller backend and enter the "My Taobao" page.

2. Find the entrances such as "Store Decoration" or "Store Settings" to enter the store's settings page.

3. On the settings page, look for relevant options such as "Shipping Settings", "Logistics Settings" or "Shipping Template".

4. There will be options about the delivery time limit on the settings page. You can set the delivery time limit to 15 days during the Spring Festival to ensure that customers are aware of the extra time required for order processing.

It should be noted that Taobao's backend settings may be adjusted according to different versions and updates, so the specific setting steps may be different. If you encounter difficulties in the operation, it is recommended that you refer to Taobao's Help Center or directly contact Taobao customer service for consultation.