There is a few ways to create new or extend a tour
1. If this is just an extension of your existing tour ie: Sunset Cruise (Wednesday to Saturday) SUMMER
You can update the date restrictions to now include Sunday:
Then in your daily rates, you will be able to stop sell all Sundays excluding January.
Then Enable Stop sells through January:
You will then need to update the tour name" Sunset Cruise (Wednesday to Saturday) SUMMER "
Alternately, you can copy and create a new tour just for sundays:
Then complete the process above.