"This day was uncommon" is wrong. You can say "Thursday was unusual in that we normally go to the theatre but we decided to go to the cinema for once." You can't say "Thursday was uncommon": there's a Thursday every week. Consider:
1) "Adders are uncommon": there aren't many adders.
2) "The design of the aeroplane was unusual": it was unconventional.
3) "He set about his work with an uncommon zeal": most people wouldn't be so zealous.
4) "It's unusual for him to stay out late": he normally comes home early.
Perhaps "uncommon" refers to the rarity of a thing and "unusual" refers to how exceptional or remarkable the thing is. I could have written "a rare zeal" in sentence (3). But the difference is not absolutely clear. You could argue that his zeal was exceptional as well as rare.