That would be an interesting day.
( That ( ( would ( be ( an ( interesting day ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (MD would) (VP (VB be) (NP (DT an) (JJ interesting) (NN day)))) (. .)))
That could be a curious day.
( That ( ( could ( be ( a ( curious day ) ) ) ) . ) )
(ROOT (S (NP (DT That)) (VP (MD could) (VP (VB be) (NP (DT a) (JJ curious) (NN day)))) (. .)))