And the Lord appeared unto Abram, and said: 'Unto thy seed will I give this land'; and he built there an altar unto the Lord, who appeared unto him.
Numbers Rabbah on Parashat Naso Parashah 10
(We are taught) That one must bring a sacrifice to God on account of good news, just as Abraham did at the hour that The Holy One blessed be He said "Unto thy seed will I give this land." Immediately, Abraham built an altar on account of the good news, as it is written "And he built there an altar unto the Lord, who appeared unto him." There is no altar without a sacrifice.
I really like what we find here. The midrash seems to be pointing out that we can learn from Abraham: we are to give a gift to God when we receive good news. How do you give God a gift? In Abraham's time there were sacrifices, but we no longer have such offerings. In truth, we see this type of gift giving to God all around us. When we make tribute donations to our synagogues in honor of a simhah, when we encourage b'nai mitzvah to commit to a mitzvah project, we are giving gifts to God in honor of good news.
It is lovely to find a custom where we celebrate our good fortune by sharing it with others. And while we are familiar with these customs, it is even nicer to think that they date back to the first Jew, Abraham, in the moment that he gets his charge here in Lekh Lekha, making it (midrashically, anyway) one of the first mitzvot. I hope we all have good news coming our way and that we remember to give gifts of celebration.