The Astronomical New Moon is on April 21, 2012 (Saturday) at 7:18 UT. On Saturday, April 21, it can not be seen anywhere in the world by naked eye. It is possible in North America that using telescope, it can be seen. On Sunday, April 22, 2012, it can easily be seen in Asia, Europe, Africa and Americas, but not in Australia (See visibility curves).
Therefore, based on Hilal (crescent) sighting by naked eye the month of Jumada-al-Thani on the islamic calendar 1433 AH would be started on Monday, 23 April 2012 for most part of the world.