added explaining concept of logical date in DAG run docs (#21433)

(cherry picked from commit 752d538)

docs/apache-airflow/concepts/dags.rst

@@ -157,6 +157,8 @@ The ``schedule_interval`` argument takes any value that is a valid `Crontab <htt
     For more information on ``schedule_interval`` values, see :doc:`DAG Run </dag-run>`.
 
     If ``schedule_interval`` is not enough to express the DAG's schedule, see :doc:`Timetables </howto/timetable>`.
+    For more information on ``logical date``, see :ref:`data-interval` and
+    :ref:`faq:what-does-execution-date-mean`.
 
 Every time you run a DAG, you are creating a new instance of that DAG which
 Airflow calls a :doc:`DAG Run </dag-run>`. DAG Runs can run in parallel for the
@@ -177,6 +179,20 @@ In much the same way a DAG instantiates into a DAG Run every time it's run,
 Tasks specified inside a DAG are also instantiated into
 :ref:`Task Instances <concepts:task-instances>` along with it.
 
+A DAG run will have a start date when it starts, and end date when it ends.
+This period describes the time when the DAG actually 'ran.' Aside from the DAG
+run's start and end date, there is another date called *logical date*
+(formally known as execution date), which describes the intended time a
+DAG run is scheduled or triggered. The reason why this is called
+*logical* is because of the abstract nature of it having multiple meanings,
+depending on the context of the DAG run itself.
+
+For example, if a DAG run is manually triggered by the user, its logical date would be the
+date and time of which the DAG run was triggered, and the value should be equal
+to DAG run's start date. However, when the DAG is being automatically scheduled, with certain
+schedule interval put in place, the logical date is going to indicate the time
+at which it marks the start of the data interval, where the DAG run's start
+date would then be the logical date + scheduled interval.
 
 DAG Assignment
 --------------

docs/apache-airflow/dag-run.rst

@@ -84,6 +84,8 @@ scheduled one interval after ``start_date``.
 
     If ``schedule_interval`` is not enough to express your DAG's schedule,
     logical date, or data interval, see :doc:`/concepts/timetable`.
+    For more information on ``logical date``, see :ref:`concepts:dag-run` and
+    :ref:`faq:what-does-execution-date-mean`
 
 Re-run DAG
 ''''''''''

docs/apache-airflow/faq.rst

@@ -214,6 +214,8 @@ This allows for a backfill on tasks that have ``depends_on_past=True`` to
 actually start. If this were not the case, the backfill just would not start.
 
 
+.. _faq:what-does-execution-date-mean:
+
 What does ``execution_date`` mean?
 ----------------------------------
 
@@ -248,6 +250,11 @@ misunderstandings.
 Note that ``ds`` (the YYYY-MM-DD form of ``data_interval_start``) refers to
 *date* ***string***, not *date* ***start*** as may be confusing to some.
 
+.. tip::
+
+    For more information on ``logical date``, see :ref:`data-interval` and
+    :ref:`concepts:dag-run`.
+
 
 How to create DAGs dynamically?
 -------------------------------