The situation
You invite an acquaintance to a weekend exhibition.
English meaning: Saturday is a bit… Sunday would work for me.
Read the Japanese original
土曜はちょっと。日曜なら空いています。The questionWhat does the reply propose?
Read the explanation
Clues in the scene
Saturday is a bit… Sunday would work for me.
A different available day is stated explicitly, so the refusal does not cover the entire invitation.
Each choice explained
Decline Saturday and offer Sunday — Best fit
Saturday is declined while Sunday is offered as an alternative. The two dates have different statuses.
Decline all weekend invitations — Does not fit
Sunday is explicitly available. Rejecting the whole weekend erases that alternative.
Attend on both days — Does not fit
Availability on Sunday does not make Saturday available too. The first part still indicates difficulty.
Leave the date entirely to you — Does not fit
The speaker supplies a specific condition, Sunday, instead of leaving the date entirely to the other person.
Same words, another scene
Two people are arranging staff to clear an event venue rather than a visit to an exhibition. The same answer gives availability, while the coordinator still needs to confirm the assignment.
Saturday remains difficult and Sunday possible, but Sunday work is not yet assigned. Availability and final allocation are separate steps.
I will check whether a Sunday assignment is possible and let you know.
Literal meaning
Saturday is difficult, but the speaker has time on Sunday.
What the expression tells us
The lesson presents 「〜はちょっと」 for declining an invitation. 「なら」 introduces a condition. Here, Sunday is an explicit alternative, so declining Saturday does not reject the whole invitation.
What these words cannot tell us
The time on Sunday, final commitment to attend and responsibility for buying tickets are not stated.
A question to ask in real life
Would visiting the exhibition at two on Sunday work for you?