Number one is more natural: I am probably talking about lunch at home on my own.
As Jonathan suggests, number two is more formal, so it would not be used in this situation of a husband talking to his wife - about leftovers...
"Tomorrow's lunch" would most likely be used in the context of an organised group lunch meeting, e.g. a group of work colleagues eating lunch together or a business lunch.
e.g. We've decided to move the venue of tomorrow's lunch to a cheaper restaurant due to budget limitations.
Obviously, in such a context, the participants would not be eating leftovers! 😉