Every other day is good! Start now though - you want to make sure you catch it! The sperm can live as long as 72 hours so every other day from today should be good.
Keep in mind that even the most regular of women can O later or sooner in any given cycle. Having said that, if you expect to O on the 21st then you should start BDing by the 15th. BD every other night until 2-3 days after your O. If you are charting you will know that you Oed when you see a temp increase for at least 2-3 days. (It will stay raised until AF comes.) The best way for you to know when to BD is to start checking your CM. If you have a lot of questions about this then you should read "Taking Charge of Your Fertility" by Toni Weschler. That should answer any questions you have.
To get pregnant with my second child we tried everything. Finally a fertility dr said to have the best chance to conceive we should have sex no more often than once a day, and to try to wait a day and a half between intercourse to give his semen enough time to rebuild supply, so I would think that if you are going to ovulate on or around the 21st, you would want to have sex the 17, 19, and 21. Doing it before you ovulate is good, because sprem can live for several days, and the journey up to the egg takes some time. Good luck. You are ovulating about the same time as me, and we are trying too. To give the sperm an extra boost we are trying pre-seed lubricant, because it is supposed to help them live longer.