There or thereabouts, explained Activision's Dan Amrich - PSN dates are out of Activision's hand, and European localisation may alter the date very slightly. 12th April is a Thursday, which is an unusual time for PSN content to appear. The US usually updates on a Tuesday, and Europe on a Wednesday.
Before then, on 29th March, PS3 premium Elite subscribers will be able to download the Overwatch multiplayer map.
All of this Modern Warfare 3 content releases first on Xbox 360. On 20th March, Collection 1 - containing content drops 1-6 - will be released for Xbox 360. There's no date on Collection 1 for PS3.
There's a Call of Duty Elite content calendar for upcoming Modern Warfare 3 content. This changes dynamically according to the platform you select.