In the old ages, there was a custom to circumcise some of the population in order to distinguish between the important people in every nation and the rest. For example, in Egypt the priests were the only one to be circumcised.

 In the Old Testament, a man can find the circumcision in some important events; the first one is after The Lord chose Abraham to go to the promise land he is telling him these words in Genesis 17:7-14-

וַהֲקִמֹתִי אֶת-בְּרִיתִי בֵּינִי וּבֵינֶךָ, וּבֵין זַרְעֲךָ אַחֲרֶיךָ לְדֹרֹתָם--לִבְרִית עוֹלָם:  לִהְיוֹת לְךָ לֵאלֹהִים, וּלְזַרְעֲךָ אַחֲרֶיךָ.  וְנָתַתִּי לְךָ וּלְזַרְעֲךָ אַחֲרֶיךָ אֵת אֶרֶץ מְגֻרֶיךָ, אֵת כָּל-אֶרֶץ כְּנַעַן, לַאֲחֻזַּת, עוֹלָם; וְהָיִיתִי לָהֶם, לֵאלֹהִים.  וַיֹּאמֶר אֱלֹהִים אֶל-אַבְרָהָם, וְאַתָּה אֶת-בְּרִיתִי תִשְׁמֹר--אַתָּה וְזַרְעֲךָ אַחֲרֶיךָ, לְדֹרֹתָם.   זֹאת בְּרִיתִי אֲשֶׁר תִּשְׁמְרוּ, בֵּינִי וּבֵינֵיכֶם, וּבֵין זַרְעֲךָ, אַחֲרֶיךָ:  הִמּוֹל לָכֶם, כָּל-זָכָר.  וּנְמַלְתֶּם, אֵת בְּשַׂר עָרְלַתְכֶם; וְהָיָה לְאוֹת בְּרִית, בֵּינִי וּבֵינֵיכֶם.  וּבֶן-שְׁמֹנַת יָמִים, יִמּוֹל לָכֶם כָּל-זָכָר--לְדֹרֹתֵיכֶם:  יְלִיד בָּיִת--וּמִקְנַת-כֶּסֶף מִכֹּל בֶּן-נֵכָר, אֲשֶׁר לֹא מִזַּרְעֲךָ הוּא.  הִמּוֹל יִמּוֹל יְלִיד בֵּיתְךָ, וּמִקְנַת כַּסְפֶּךָ; וְהָיְתָה בְרִיתִי בִּבְשַׂרְכֶם, לִבְרִית עוֹלָם. וְעָרֵל זָכָר, אֲשֶׁר לֹא-יִמּוֹל אֶת-בְּשַׂר "עָרְלָתוֹ--וְנִכְרְתָה הַנֶּפֶשׁ הַהִוא, מֵעַמֶּיהָ:  אֶת-בְּרִיתִי, הֵפַר

" I will establish my covenant as an everlasting covenant between me and you and your descendants after you for the generations to come, to be your God and the God of your descendants after you. The whole land of Canaan, where you now reside as a foreigner, I will give as an everlasting possession to you and your descendants after you; and I will be their God.”"Then God said to Abraham, “As for you, you must keep my covenant, you and your descendants after you for the generations to come. This is my covenant with you and your descendants after you, the covenant you are to keep: Every male among you shall be circumcised. You are to undergo circumcision, and it will be the sign of the covenant between me and you. For the generations to come every male among you who is eight days old must be circumcised, including those born in your household or bought with money from a foreigner—those who are not your offspring. Whether born in your household or bought with your money, they must be circumcised. My covenant in your flesh is to be an everlasting covenant. Any uncircumcised male, who has not been circumcised in the flesh, will be cut off from his people; he has broken my covenant"
 

The circumcision is a covenant between The Lord and men of this world.  Once the man loses his foreskin, he shows his loyalty to The Lord not just in his soul but also in his body. The circumcision is a physical sign of a covenant between G-d and his people.

Another important issue is the time. Why after eight days?
Some say that the baby is too small to say no. In Genesis Rabbah, there is a saying that every baby must enjoy at least one Shabbat before he becomes a side in the covenant with The Lord.

The second appearance of the circumcision is in the story of the rape of Dinah. In Genesis 34 we are reading that the brothers of Dinah told the people of Nablus these words:

"וַיֹּאמְרוּ אֲלֵיהֶם, לֹא נוּכַל לַעֲשׂוֹת הַדָּבָר הַזֶּה--לָתֵת אֶת-אֲחֹתֵנוּ, לְאִישׁ אֲשֶׁר-לוֹ עָרְלָה:  כִּי-חֶרְפָּה הִוא, לָנוּ. אַךְ-בְּזֹאת, נֵאוֹת לָכֶם:  אִם תִּהְיוּ כָמֹנוּ, לְהִמֹּל לָכֶם כָּל-זָכָר

"They said to them, “We can’t do such a thing; we can’t give our sister to a man who is not circumcised. That would be a disgrace to us. We will enter into an agreement with you on one condition only: that you become like us by circumcising all your males" (Genesis 34:14-15)

Here we can learn that the circumcision is a sign that helps us to differentiate between two groups, the sons of Jacob and the others. This circumcision created an enormous pain in the people of Nablus and enabled Simeon and Levi to kill all the people of the city, because one of them raped their sister.

The next story with circumcision is the one who saved Moses' son from a certain death. The one who did the circumcision is zipporah
,as written in Exodus 4:24-26-

וַיְהִי בַדֶּרֶךְ, בַּמָּלוֹן; וַיִּפְגְּשֵׁהוּ יְהוָה, וַיְבַקֵּשׁ הֲמִיתוֹ.  וַתִּקַּח צִפֹּרָה צֹר, וַתִּכְרֹת אֶת-עָרְלַת בְּנָהּ, וַתַּגַּע, לְרַגְלָיו; וַתֹּאמֶר, כִּי חֲתַן-דָּמִים אַתָּה לִי.  וַיִּרֶף, מִמֶּנּוּ; אָז, אָמְרָה, חֲתַן "דָּמִים, לַמּוּלֹת
 

"And it came to pass on the way at the lodging-place, that the LORD met him, and sought to kill him. Then Zipporah took a flint, and cut off the foreskin of her son, and cast it at his feet; and she said: 'Surely a bridegroom of blood art thou to me.'So He let him alone. Then she said: 'A bridegroom of blood in regard of the circumcision.'

The story reveals to us that the leader of Israel, Moses, must keep the covenant between The Lord and the people of Israel in order to let his people go out from Egypt.

The next stop is after the entering to the promise land. Joshua, the new leader must circumcise all the people there, as we can read in Joshua 5:2-8-,

" בָּעֵת הַהִיא, אָמַר יְהוָה אֶל-יְהוֹשֻׁעַ, עֲשֵׂה לְךָ, חַרְבוֹת צֻרִים; וְשׁוּב מֹל אֶת-בְּנֵי-יִשְׂרָאֵל, שֵׁנִית.  וַיַּעַשׂ-לוֹ יְהוֹשֻׁעַ, חַרְבוֹת צֻרִים; וַיָּמָל אֶת-בְּנֵי יִשְׂרָאֵל, אֶל-גִּבְעַת הָעֲרָלוֹת.  וְזֶה הַדָּבָר, אֲשֶׁר-מָל יְהוֹשֻׁעַ:  כָּל-הָעָם הַיֹּצֵא מִמִּצְרַיִם הַזְּכָרִים כֹּל אַנְשֵׁי הַמִּלְחָמָה, מֵתוּ בַמִּדְבָּר בַּדֶּרֶךְ, בְּצֵאתָם, מִמִּצְרָיִם.  כִּי-מֻלִים הָיוּ, כָּל-הָעָם הַיֹּצְאִים; וְכָל-הָעָם הַיִּלֹּדִים בַּמִּדְבָּר בַּדֶּרֶךְ, בְּצֵאתָם מִמִּצְרַיִם--לֹא-מָלוּ.  כִּי אַרְבָּעִים שָׁנָה, הָלְכוּ בְנֵי-יִשְׂרָאֵל בַּמִּדְבָּר, עַד-תֹּם כָּל-הַגּוֹי אַנְשֵׁי הַמִּלְחָמָה הַיֹּצְאִים מִמִּצְרַיִם, אֲשֶׁר לֹא-שָׁמְעוּ בְּקוֹל יְהוָה:  אֲשֶׁר נִשְׁבַּע יְהוָה, לָהֶם, לְבִלְתִּי הַרְאוֹתָם אֶת-הָאָרֶץ אֲשֶׁר נִשְׁבַּע יְהוָה לַאֲבוֹתָם לָתֶת לָנוּ, אֶרֶץ זָבַת חָלָב וּדְבָשׁ.  וְאֶת-בְּנֵיהֶם הֵקִים תַּחְתָּם, "אֹתָם מָל יְהוֹשֻׁעַ:  כִּי-עֲרֵלִים הָיוּ, כִּי לֹא-מָלוּ אוֹתָם בַּדָּרֶךְ.  וַיְהִי כַּאֲשֶׁר-תַּמּוּ כָל-הַגּוֹי, לְהִמּוֹל; וַיֵּשְׁבוּ תַחְתָּם בַּמַּחֲנֶה, עַד חֲיוֹתָם.
 

"At that time the LORD said to Joshua, “Make flint knives and circumcise the Israelites again.”So Joshua made flint knives and circumcised the Israelites at Gibeath Haaraloth. Now this is why he did so: All those who came out of Egypt—all the men of military age—died in the wilderness on the way after leaving Egypt. All the people that came out had been circumcised, but all the people born in the wilderness during the journey from Egypt had not. The Israelites had moved about in the wilderness forty years until all the men who were of military age when they left Egypt had died, since they had not obeyed the LORD. For the LORD had sworn to them that they would not see the land he had solemnly promised their ancestors to give us, a land flowing with milk and honey. So he raised up their sons in their place, and these were the ones Joshua circumcised. They were still uncircumcised because they had not been circumcised on the way. And after the whole nation had been circumcised, they remained where they were in camp until they were healed"

For me the most important circumcise that appears in The Old Testament is not the physical but the mental, the circumcision of the heart, as we read in Deuteronomy 30:6-

"וּמָל יְהוָה אֱלֹהֶיךָ אֶת-לְבָבְךָ, וְאֶת-לְבַב זַרְעֶךָ:  לְאַהֲבָה אֶת-יְהוָה אֱלֹהֶיךָ, בְּכָל-לְבָבְךָ וּבְכָל-נַפְשְׁךָ--לְמַעַן חַיֶּיךָ."
 

"The LORD your God will circumcise your hearts and the hearts of your descendants, so that you may love him with all your heart and with all your soul, and live"
 
Hearing those words that are really similar to "Shema Israel" is the covenant that The Lord had, has and will have with us, so we need to listen to Jeremiah's words and maintain them in our heart:

כִּי-כֹה אָמַר יְהוָה, לְאִישׁ יְהוּדָה וְלִירוּשָׁלִַם, נִירוּ לָכֶם, נִיר; וְאַל-תִּזְרְעוּ, אֶל-קֹצִים.  הִמֹּלוּ לַיהוָה, וְהָסִרוּ עָרְלוֹת לְבַבְכֶם, אִישׁ יְהוּדָה, וְיֹשְׁבֵי יְרוּשָׁלִָם:  פֶּן-תֵּצֵא כָאֵשׁ "חֲמָתִי, וּבָעֲרָה וְאֵין מְכַבֶּה--מִפְּנֵי, רֹעַ מַעַלְלֵיכֶם

"This is what the LORD says to the people of Judah and to Jerusalem:
 “Break up your unplowed ground
 and do not sow among thorns.
 Circumcise yourselves to the LORD,
 circumcise your hearts,
 you people of Judah and inhabitants of Jerusalem,
or my wrath will flare up and burn like fire
 because of the evil you have done—
 burn with no one to quench it"

 Have a great week, eli