Microsoft officials sent invitations on December 11 to selected press and analysts to its next Windows 10 reveal event.

The keynote from the day-long event on January 21 will be live streamed. Microsoft CEO Satya Nadella, Windows execs Terry Myerson and Joe Belfiore, and Xbox chief Phil Spencer will be speaking. The event will focus on the "Windows 10 consumer experience," a spokesperson confirmed.

My tipsters had suggested that the coming preview event would be in Redmond on January 20 and 21.

The invitation doesn't say that Microsoft will be showing off for the first time in public the combined Windows Phone-Windows RT Windows 10 mobile SKU that is expected to work on Intel- and ARM-based tablets, as well as Windows Phones. But that is on the agenda, my sources have said. That version of Windows 10 is not going to include a desktop, and will run Windows Store/"Metro Style" apps only, sources have said.

Microsoft also is expected by those same sources to show off the January Technical Preview (JTP) of Windows 10, which will be the next test release of Windows 10 that the company will make available to its "Windows Insider" testers. In mid-November, Microsoft released its second publicly available Windows 10 test build update (9879), which officials said would be the last Windows 10 Technical Preview update the company would deliver officially this calendar year.

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The coming JTP is expected to include support for Continuum, a feature that will allow users with two-in-one devices to more easily transition modes when connecting and disconnecting keyboards. It also may be the first build where Microsoft turns on Cortana integration in Windows 10.

I'm not sure exactly when Microsoft will make available to external testers the first Windows 10 mobile SKU test build. I'm hearing that Microsoft is still testing the mobile SKU on a very limited basis within the Operating Systems Group at this point. I would think the mobile SKU might need to be tested internally at Microsoft more widely before Microsoft releases it to external testers.

But once Microsoft does deliver the Windows 10 mobile bits to testers, ongoing and regular updates will be coming at least monthly, according to sources.

Microsoft's first Windows 10 reveal event for press and analysts was on September 30. The Operating Systems Group made the first test builds of Windows 10 client and server available the day after that event, on October 1.

During the September event, Microsoft officials said to expect Windows 10's "launch wave" to hit around mid-2015. More recently, Chief Operating Officer Kevin Turner said Windows 10 would launch between late summer and early fall 2015. (Remember: Launch events are typically more orchestrated marketing events than dates when Microsoft makes Windows bits available to its OEM and licensed customers.)

I'm expecting Microsoft to RTM all of its Windows 10 SKUs simultaneously, meaning Windows 10 for PCs/desktops; Windows 10 mobile for tablets and phones and Windows Server Next (likely Windows Server 2015) would be deemed "done" on the same date. Microsoft will likely continue to update the Windows 10 PC and mobile SKUs with new and updated features and fixes even after the product hits RTM, as it has done with Windows 8 and more recent Windows operating system releases.