This concerns the period sometimes called the "Dark Age" in Tibetan history. The "first spread" (snga dar) of Buddhism dates from the Imperial period, in which Padmasambhava, King Trisong Detsen, and Shantarakshita are supposed to have played the key roles. The Nyingma traditions trace their origin to this period. The Tibetan empire collapsed in the 840's. Phyi dar refers to the "later spread" of Buddhism, beginning somewhat more than a century later. The Sarma Schools trace their origin to the phyi dar.
The "Dark Age," between the first and later spreads, is "dark" in that historical records are scarce, so we can't see into it easily. It is also supposed to have been "dark" in the sense that Buddhism "declined" and almost died out.
However, recent research suggests that Tantric Buddhism flourished during this period; it is monasticism that declined. This makes the period of perhaps great interest to non-monastic Tantrikas. (More about that in future posts.) In the past few years, historians have paid increasing attention to the Dark Age, and they are finding intriguing new sources of evidence.
As far as I know, this will be the first conference devoted specifically to the period.
Fortunately, for those of us not qualified or able to attend, a proceedings (collection of presentations) will be published. I am very much looking forward to reading it.
[Update: the conference page now includes extended abstracts of the papers given at the meeting.]