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After months of anticipation and a mix of official and internally communicated delays, Microsoft’s long-awaited Windows 10 Mobile upgrade is on the eve of being deployed. Playing its cards incredibly close to the chest, the company will announce its same-day availability on March 17, according to a person familiar with the company’s plans.

As previously reported by VentureBeat in a story pointing to March as the latest intended rollout target, legacy in-market devices selected to receive the upgrade will at first need to initiate it manually. This is known as the pull model (which is distinct from a Microsoft-triggered push model, which will follow at an undisclosed date).

The specific Nokia- and Microsoft-branded Lumias approved for the update are an even more closely guarded secret, and will not be revealed until a Microsoft announcement tomorrow, according to the briefed individual.

After missing a publicly messaged December rollout target, as well as an internal one in February, this update for Windows Phone 8.1 handsets has become something of a white whale for Windows fans. Tomorrow, the whale will finally breach.

Update on March 17: As promised, Windows 10 Mobile is now rolling out.