As described, you are presumably attempting to disable WiFi from the iPad Control Centre. By design, disabling WiFi/Cellular/Bluetooth radios from the Control Centre is temporary; radios disabled in this way will automatically re-enable the following day as described.
If you wish to manually disable a network radio until you next wish to use it, you should disable the related radio from iPad settings. For example, to fully disable WiFi:
Settings > WiFi > WiFi - set the toggle to OFF
Be aware that leaving your WiFi enabled on your iPad should not excessively drain your iPad battery - unless the iPad is unable to find a registered WiFi network to which it may connect. If the iPad is continuously searching for a WiFi network, battery utilisation may be a greater that normal.