Kinda related with SAT - I waited until March of my junior year to take the SAT..
For the ACT, I would strongly advise taking it in February! It gives you so much more flexibility in my opinion. I didn't realize all the testing/stuff I had to do by the end of the year, and I wish I had taken the January SAT...
February - ACT
January/March - SAT if you want to try it
April - ACT retake
May - APs: you will be tired
June - probably SAT 2s: most schools require some - check beforehand
Senior Year - HOPEFULLY NO TESTING!!!
However, like amaster008 said, you may get a lower score in Feb. Even if this is true, you will have a real testing experience under your belt and know what you can improve on! Plus, by taking it earlier, you have more flexibility with when you can take it! You can choose April/June! Good luck!