By Rabbi Berach Steinfeld
In the beginning of  Parshas Vayechi, Yaakov Avinu blessed his grandchildren, Ephraim and Menashe. He told them that Yisroel will bless their children to be like Ephraim and Menashe. The posuk uses the words “on that day” referring to when Yaakov bentched them.
The Targum Yonoson writes that this bracha shall be given to each child on the day he is circumcised.
The Pirush Yonoson explains the specificity of the day of milah as the time of this bracha since the posuk uses the term “on that day.” He adds that the word Yisroel seems to be unnecessary in the posuk since it is obvious that a Yisroel (not a goy) will bentch his child in this way. By using the word Yisroel, the posuk is teaching us that the day the child becomes a Yisroel is the day he receives this bracha.
The Moshav Zekeinim Le’Baalei Tosfos explain that the Sefardim who say the following upon completion of the Bris carry out this tradition. “It should be the will of Hashem that the baby should be “Ach l’Shiva and also l’shmona.” This is saying that the baby should be like Ephraim and Menashe who were numbers seven and eight when the Nesiyim brought the korbanos at the inauguration of the Mishkan.
The question arises; why is it important to give this Bracha on the day of a Bris over any other day?
The first answer is that there is a great ayin hora on the day of the Bris Milah. (We learn that Bilam tried harming Bnei Yisroel when they were in the midbar on the day of the milah.) We therefore bless the child that he should be like Ephraim and Menashe who were from Shevet Yosef, and therefore an ayin hora has no effect on them.
Secondly, we want to give a child the bracha as early as possible since it might be too late once the child is grown up, so we bracha is said as soon as he enters into the covenant of being a Jew.
The last answer is that we know that when a child cries at the bris all tefillos go up to shomayim without any hindrance. That is why we pick that opportune time to give the baby this bracha.
Let us hope that all these tefillos and brachos will be mekuyam.