Seasonal Variations in the Barrier

The Void is the metaphysical remains of Turoch's rage and hunger and the Barrier was erected by the Nine to keep the Void from destroying the mortal plane.
The Barrier is a mighty wall of divine wind, but it is not perfect and has many small holes which pop up for short periods and then disappear. Void demons prowl the Barrier and look for holes that can slip through to enter the material plane.
On the Winter Stellar Day, the Nine's power is collectively weakest because it is the anniversary of the Divine Rebellion and on the Summer Stellar Day, the Nine's power is collectively the strongest because that day is far removed from the anniversay of the Divine Rebellion.
This means every summer the Barrier gets stronger and every winter the Barrier gets weaker. With this fluctuation, most Void demons who escape the Void do so in the winter. As a demon becomes stronger, they also become bigger, so stronger demons require bigger holes in the Barrier to escape. The demon subtype known as winter demons are believed to only be able to escape during the winter.

Manifestation

Without some kind of enhancement spell, a mortal usually cannot see a gap in The Barrier as the Barrier is made of wind and mortals usually cannot see the winds.
Void Demons have a different visual spectrum known as lifesight and can see the Barrier clearly and look for weak spots this way. Unfortunately for them, gaps in the Barrier are usually very temporary so while Void demons have long range telepathy by the time they tell their fellows "there is a gap over here!" and their companions fly to the gap, it is probably closed when they get there.
During the winter months, gaps are more frequent, larger, and longer lasting, letting more demons escape.

Localization

During the Red Era, there was a notable uptick of demon attacks every winter and spring.
This pattern has gradually faded away. Partially because the Barrier is slightly stronger in the Feudal Era than the Red Era and partially because it seems the Void demons are changing their tactics.
They still cross the Barrier more often during the winter months, but they no longer stage an attack on the material plane immediately and they no longer attack the lands closes to the poles first every time.

This is an extension of

Type

Metaphysical, Demonic