HTC is planning to unveil two new Windows Phone 7.5 “Mango” devices on September 1, according to reports.

Microsoft stoked the rumor flames on Monday after the official UK Windows Phone Facebook account announced HTC’s party on September 1. Gadget site BGR claims that HTC is launching the Eternity and Omega Windows Phone devices. The HTC Eternity is a Windows Phone 7.5 device with a 3.7-inch display. HTC is also preparing the HTC Omega, a massive 4.5-inch Windows Phone 7.5 device. Both devices are expected to launch on American carrier networks so it’s unclear whether the same devices will be unveiled at the London HTC event.

Commonly referred to as Windows Phone Mango, its codename, Windows Phone 7.5 was finalized by the software giant in late July. Microsoft is planning to launch a number of Windows Phone 7.5 devices on September 1, according to reports. The date ties in with IFA, Europe’s largest consumer electronics show in Berlin and should see the unveiling of new devices from Samsung, Acer, ZTE and LG. WinRumors understands that Microsoft will announce several Windows Phone 7.5 devices on September 1 but that the update for existing devices will not be made available immediately. Microsoft is understood to be preparing a rolling update instead of a single “big bang” approach. Windows Phone developers will be able to submit 7.5 applications later today alongside the release of the Windows Phone Mango RC development tools. Microsoft will likely wait until its development tools are fully baked and OEMs/carriers sign off on Mango before pushing the update to existing devices.

Microsoft’s Nokia partnership will likely be strengthened by the launch of its first Windows Phone 7.5 device in October. Nokia is understood to be planning a launch party event for its Windows Phone device at Nokia World on October 26. The majority of new Windows Phone 7.5 devices will be made available in time for the important holiday season.