I think the lesson behind Marcheshvan is that we need to infuse everyday life with holiness and impart everyday life with a sense of what we should have learned over the past couple of months. Elul should teach us the necessity of asking G-d’s forgiveness and selicha. Rosh HaShanah should impart in us a sense of awe of G-d’s Kingship and Dominion. Yom Kippur should remind us of the need to constantly seek to do teshuva and amend and improve our ways. Sukkot is a reminder of the passing and fleeting nature of the world and its inherent follies (Kohelet). Hoshana Rabba is a reminder to again to improve the world and make permanent changes in it. Shemini Atzeret is to remind us of the need to internalize what symbols are supposed to represent to us, and to focus on our inner selves and experiencing inner joy, Simchat Torah is a reminder of the need to always delight and rejoice in the Torah, and Shabbat Bereshit is a reminder of the need to be focused on our renewal and on the ever-turning cycle of creation and Torah.
Our task is to incorporate these psycho-spiritual messages into our everyday lives and to elevate the mundane.