Gandalf
Gandalf's durability is empirically exceptional. Having literally died in combat with a creature of shadow and flame, he was subsequently returned to life by the Valar with enhanced capabilities and upgraded robes. This represents a durability rating that transcends conventional mortality metrics.
Moreover, his existence predates the creation of Arda itself, making him approximately several billion years old by conservative estimate. He has survived the machinations of Sauron, the corruption of Saruman, and countless attempts on his existence by Orc, Nazgul, and irritable Hobbit alike.
Monday
Monday's durability derives from a fundamentally different source: it is a concept rather than an entity. One cannot kill Monday any more than one can stab arithmetic or wound the colour blue. Attempts to abolish Monday through calendar reform have historically resulted merely in Tuesday acquiring Monday's characteristics.
The seven-day week has persisted across civilisations for over 4,000 years, surviving the fall of empires, world wars, and the invention of the weekend. Monday emerges from each challenge precisely as it was, unchanged and unchangeable.