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.
BUT (and this is a big but), you should never apply undiluted tea tree oil to acne. The potent solution can make skin become dry, irritated and itchy, which is the last thing you want when you're suffering from this stubborn condition. The best thing to do is mix 5ml of tea tree oil with 95 ml of water. Apply as a toner over acne prone areas. Try mixing tea tree with aloe vera gel and honey too - the ways in which you can use this nifty solution are endless.