If the iPad event was going to happen in late March, the invites should of been sent out today. When the iPad event was in late March the invites were sent out on the 10th.
Maybe the rumour of early April is accurate, then invites should be sent out some time next week.
Those invites usually come out the week before. The cadence (from what I've seen err... recollect):
Monday, 8am PST: Invitations go out with some tagline for the following week's Tuesday
Monday, 8:00:00.0002: Macrumors has a post, everyone starts speculating what the tagline means, and how all problems in their lives will be exacerbated by waiting "forever" for the event, then more kvetching on the delay until the orders can be placed. Lines start forming at every Apple Store on the planet.
Monday, 10:30am: Samsung schedules an event on the following Monday with their next device
Monday next, 11:59pm: All of the rumors are now one thousand fold, and new device will prove intergalactic life can (and will) talk to us on the new device.
Tuesday, 10am, the event begins with Tim in a video doing something lovable, but dorky
Tuesday, 11:45am, 99.9% of the Macrumors readers are both hating the reality of the device not living up to unrealistic expectations, and also secretly having their clocks set to midnight Friday, when they can order it.
Friday, 8:00am, Apple Employees handing out danishes to the line at the stores, worldwide. The day before, everyone watched the New Zealand and Australia feeds for the totally awesome new device, and are now wiping drool off of their faces at 5 minute intervals
Friday, 9am, the store opens, and people are simultaneously ecstatic and tired.
Well, that's my experience.... YMMV