Heidegger's interpretation of DASEIN purports to show it ‘as it is firstly and mostly [zunächst und zumeist] in its average everydayness [Alltäglichkeit]’ (BT, 16). Tag, ‘day’, and all, ‘all, every’, furnish: Alltag, ‘weekday; everyday life’; alltäglich, ‘daily; everyday, ordinary’; and Alltäglichkeit, ‘everydayness, ordinariness’. In contrast to a ‘distinctive, definite way of existing’, everydayness is Dasein's ‘indifferent Firstly and Mostly [Zunächst und Zumeist]’. Since it is so close to us, it is usually overlooked: ‘What is ontically closest and most familiar is what is ontologically furthest, unknown and constantly overlooked in its ontological meaning’ (BT, 43). Alltäglichkeit ‘clearly means that mode of existing which Dasein observes “every day” [‘alle Tage’].’ But everydayness is not a quantitative concept: ‘“every day” does not mean the sum of the “days” allotted to Dasein in its “lifetime”.’ It means ‘a definite How [Wie] of existence that pervades Dasein “for life”.’ That is ‘the How in which Dasein “lives from day to day”, whether in all its conduct or only in certain conduct prescribed by being-with-one-another’ (BT, 370). Everydayness contents itself with the habitual, even when this is burdensome. It is uniform, but it finds variety in whatever the day brings. Everydayness is inescapable. . .
I had forgotten about the importance of everydayness for Heidegger when I started this blog, but now realize that it's the perfect title. Anyways, cheers.