Variously called Death Day, Decision day, and a number of other less kind words, D-Day was the day that the Allies began the big push from the coast of France all the way to the heart of Germany, ending WWII.
More importantly when it comes to cystic acne, tea tree oil can be harmful to your skin when applied in high concentrations. Some individuals have reported severe skin irritation and allergic dermatitis after applying too much tea tree oil, or applying the proper dose too frequently.
Tea tree oil has been used traditionally as a topical antiseptic and antifungal treatment. One study found that a dilution of tea tree oil worked as well as 5% benzoyl peroxide in controlling the symptoms of acne. It appears to be effective with toenail fungus and possibly athlete's foot.