"Vateitze Yitzchok Lasuach basodeh lifnos erev" (Breshis 24:63)
Chazal (Berachos 26b) derived from this passuk that Yitzchak instituted tefilas Mincha. Tefilas Shacharis had already been instituted by Avraham and later in Parshas Vayeitze, Yaakov will establish tefilas Arvis. Each of the tefilos of the avos corresponded to the events in their lives and to the relationships they had with their contemporaries.
Avraham was treated with great respect by those around him. He is referred to as a nesi Elokim - a prince of Hashem. He is victorious in battle against the four powerful kings and is sought after by Avimelech to enter into a treaty. It was appropriate for Avraham to establish the tefilah recited in the morning, since the sun shone on Avraham as he personified the strength and vigor of one beginning a new day.
Yitzchok had a more difficult relationship with those around him. He is envied, despised and eventually told to leave the land of Gerar. His life is not in danger, yet the sun that had once shone over his father is obviously about to set. Tefilas Mincha, which is recited as night is approaching is the legacy of Yitzchok.
Yaakov lives in a time of darkness. He escapes death at the hands of Esav only to suffer in the house of Lavan. Subsequently his life was once again in danger after fleeing from Lavan. Yaakov, whose very existence is synonymous with night, was the first to recite tefilas Maariv.
Following Chazal who saw in the lives of the avos foreshadowing of later events in Jewish history, the three tefilos of the avos set the stage for three distinct periods in our history. Chazal (Rosh Hashana 18b), while discussing the status of fast days associated with the destruction of the Beis Hamikdash, divide Jewish history into three categories. During times of peace, these days become times of celebration. When there are decrees of persecution, it is obligatory to fast on these days. At times of neither complete peace nor persecution, these fast days are voluntary. Although we have accepted to fast on these days even during times of no outright persecution, technically, these three categories govern the status of these days as actual fast days.
Each distinct era in our history was foreshadowed by the lives and tefilos of the avos. Avraham taught us to daven during times of peace, whereas Yaakov is our model during times of darkness. We emulate Yitzchak when we are somewhere in between the morning of Avraham and the night of Yaakov.
The tefilah of Yitzchok is perhaps the most difficult one. Dovid Hamelech describes Tehillim two times of tefillah: "kos yeshu'os esah u'vesheim Hashem ekra" during times of great salvation I call upon Hashem and "tzarah v'yagon emtza u'b'sheim Hashem ekra" - during times of great distress I call upon Hashem. It is fairly simple to approach Hashem during these times. Feelings of gratitude or fear encourage one to turn to Hashem through tefillah. It is much more difficult to do so when one is neither in a state of intense joy nor in great despair.
Most of our lives are neither moments of kos yeshu'os esah nor tzarah v'yagon emtza. Both as individuals and as a nation we are faced with the challenge to maintain our connection to Hashem during the "regular" days. The times when the sun is not necessarily shining so brightly, yet not quite dark are the times when we tend to neglect our obligation of tefillah. It is the legacy of Yitzchok to turn to Hashem even during these times.