The Ice Plains is the place of Jaques' visions, to which the Grand Master teleports Geralt in the Epilogue, during the Frozen Reflections quest, as he attempts to convince him that what he has done is all for the greater good. Regardless of the time of day, this area is always in full daylight.
It should be noted that potions last much longer here as time is slowed by about ten times. 4 hour effects become 40 hour effects. Any potions taken prior to traveling here will also be affected.
Avallac'h confirms the White Frost is the ultimate fate of one of the worlds he and Geralt travel through on the journey to Tir ná Lia. All worlds face this demise of endless winter with each one at a different stage in its journey towards eternal permafrost.
Based on a report picked up in Tir ná Lia itself, the White Frost has claimed many regions of the Aen Elle homeworld with its mages struggling to buy time for the Aen Elle to launch full-scale invasions of Aen Seidhe worlds for survival.