The authentic historicity of Dasein "has a meaning defined by our own personal past" and future intentions. The inauthentic historicity does not.
Being & Time, H.376, page 428 (Macquarrie & Robinson trans.)
In accordance with the way in which historicality is rooted in care,
Dasein exists, in each case, as authentically or inauthentically
historical.
Authentic historicity is, by definition, one's personal (proper to oneself) past, present and future.
Dasein becomes authentic when it overtakes its thrownness.
A few supporting notes
H. 303, page 351
Ontologically, Dasein is in principle different from everything that
is present-at-hand or Real. Its 'subsistence' is not based on the
substantiality of a substance but on the 'Self-subsistence' of the
existing Self, whose Being has been conceived as care. The phenomenon
of the Self — a phenomenon which is included in care — needs to be
defined existentially in a way which is primordial and authentic, in
contrast to our preparatory exhibition of the inauthentic they-self.
H. 381, page 433
We contend that what is primarily historical is Dasein. That which
is secondarily historical, however, is what we encounter
within-the-world — not only equipment ready-to-hand, in the widest
sense, but also the environing Nature as 'the very soil of history.'
Entities other than Dasein which are historical by reason of belonging
to the world, are what we call 'world-historical'. It can be shown
that the ordinary conception of 'world history' arises precisely from
our orientation to what is thus secondarily historical.
H. 382, page 434
Dasein understands itself with regard to its potentiality-for-Being,
and it does so in such a manner that it will go right under the eyes
of Death in order thus to take over in its thrownness that entity
which it is itself, and to take it over wholly. The resolute taking
over of one's factical 'there', signifies, at the same time, that the
Situation is one which has been resolved upon.
So Dasein starts out as thrown with the "they" within-the-world, inauthentically. Then takes over itself with resolution into authenticity. Authentic temporality is finite: birth to death. It is futural — planning ahead, based on its (personal) past.
H. 383, page 435
As thrown, Dasein has indeed been delivered over to itself and to its
potentiality-for-Being, but as Being-in-the-world. As thrown, it has
been submitted to a 'world', and exists factically with Others.
Proximally and for the most part the Self is lost in the "they". It
understands itself in terms of those possibilities of existence which
'circulate' in the 'average' public way of interpreting Dasein today.
These possibilities have mostly been made unrecognizable by ambiguity;
yet they are well known to us. The authentic existentiell
understanding is so far from extricating itself from the way of
interpreting Dasein which has come down to us, that in each case it is
in terms of this interpretation, against it, and yet again for it,
that any possibility one has chosen is seized upon in one's
resolution.
H. 385, page 437
Only an entity which, in its Being, is essentially futural so that it is free for its death and can let itself be thrown back upon its
factical "there" by shattering itself against death — that is to say,
only an entity which, as futural, is equiprimordially in the process
of having-been, can, by handing down to itself the possibility it has
inherited, take over its own thrownness and be in the moment of vision
for 'its time'. Only authentic temporality which is at the same time
finite, makes possible something like fate — that is to say, authentic
historicality.
