Yahrtzeit- Jewish concept or secular?

Shalom and thank you for your question! You want to understand why a Yahrtzeit should be commemorated on the Jewish date. May I assume that you are referring to the Yahrtzeit of a Jewish person?

If so, then we need to understand why we have a Jewish calendar at all, what can make a calendar Jewish,  and a person Jewish…

A person is Jewish if born to a Jewish mother, or if he or she underwent a conversion according to the requirements of Jewish law,  (Halacha) out of a sincere desire to accept the Jewish way of life and study the Torah and keep the mitzvot.

Abraham was the first ‘Hebrew’. The word for Hebrew in Hebrew is ‘Ivri’. It is connected to the root ‘side’ – ‘ever’ in Hebrew. Abraham was ideologically on one side, believing in One G-d, while the people of the idol-worshipping generation he lived in were on the ‘other side’. Fast forward to Abraham’s descendants when they are about to be redeemed from Egypt after years of slavery. G-d has appointed Moses as the leader that will lead the Jews out of Egypt and into the desert where they will receive the Torah, learn it and carry out its commandments, thus becoming the Chosen Nation. (Chosen to be a light to the world. Indeed the belief in a G-d that we cannot see, as opposed to idols, is a belief that was adopted by much of the world, and many  of the systems of civil law are based on Biblical law.)

Just before G-d tells Moses what to tell the people regarding the Pascal sacrifice,  which was the last thing they would do before leaving Egypt, G-d shows Moses the new moon, and tells him that this is what he should see and sanctify every month. In Exodus chapter 12:2 it says:

“This month shall be to you the head of the months; to you it shall be the first of the months of the year.” This was the beginning of the Jewish calendar. Commentaries raise the question of why this was the first commandment given to the Jewish people. Remember,  this is before trekking into the Sinai desert and receiving the first of the Ten Commandments from G-d Himself and then being taught the rest of them and the rest of the Torah and it’s details by Moses.

G-d showed Moses the NEW moon. The moon is an entity that constantly renews itself, every month. Before we embarked on our spiritual journey of receiving the Torah and thus on our appointed task in this world,  G-d wanted us to know that we must beware of falling into a rut, and remaining in our comfort zone. We must be willing to constantly reassess and go forward…

Everything that our forefathers went through left its mark on our national psyche. Brain scientists know that if a person goes through a traumatic experience or an intense one at some point,  his or her brain will tend to replay thoughts or feelings from that experience on the same date a year later. This works on a spiritual level for the nation as well. There is a spiritual consciousness that replays itself every year when we celebrate the exodus from Egypt on Passover, for example, or accept upon ourselves the Kingship of G-d on Rosh HaShanah, the Jewish New Year.

Thus, a Jewish person is connected by his soul to the national spiritual consciousness, and the Hebrew birth date is the one that has significance for his soul.