Earlier in the week we reported that the development of the Windows Phone 8.1 core had finished. There were some minor bugs by the secondary team that still needed to be ironed out, but in essence, the OS is completely wrapped.

What wasn’t clear was when the OS would be dropped to OEM partners to be put on devices. We assumed that it would be after some of those minor tweaks had been ironed out, but as it turns out it was March 26. In other words, the final build of Windows Phone 8.1 is now in Microsoft partners’ hands, being tested and prepped for devices.

That leads us to some other questions, like scheduling. Through various sources we’ve been able to verify that starting Wednesday, April 23rd, new phones running Windows Phone 8.1 will be available. It’s not clear which phones those are, but presumably the Lumia 630, Lumia 930 and Samsung ATIV SE (for Verizon) are top contenders. We’re also supposing that this is for US markets although it’s not known at this time.

Likewise for the much anticipated Windows Phone 8.1 Developer Preview. That program, which is open to anyone with a free App Studio account, is expected to be rolled out by ‘late April’. We’re not aware of a firm date yet for that program, but it is in its final stages of preparation.

The Preview for Developers program will allow anyone with an App Studio account and the Developer Preview app to download and run the final 8.1 bits on their Windows Phone 8 device – much earlier than waiting official updates. While technically for developers, it’s a wink and a nod to the ‘enthusiast community’, who want 8.1 just weeks after its announcement.

Finally, as to when those official over-the-air (OTA) updates will go out, it’s not clear. Official updates for individual phones are slightly more complicated due to the corresponding firmware that needs to be developed and the added hurdle of carrier testing. Ultimately, it is the carriers who will roll out the updates and like previous ones that could take months. Luckily if you want Windows Phone 8.1, you can have it within the next 30 days.

Windows Phone 8.1 is presumed to be announced next week in San Francisco, during Microsoft’s Build conference. We’ll be on hand to cover it live.