"And God blessed them, and God said to them, Be fruitful and multiply" (Genesis 1:28). Bar Kappara taught: A virgin marries on the fourth day [Wednesday] and is wed on the fifth [Thursday], since on it [the fifth day of creation] a blessing was stated for the fish. A widow marries on the fifth day and is wed on the sixth, since on it a blessing was stated for the man [Adam]. Should a widow too be wed on the fifth, since a blessing was stated on it for the fish? The blessing of the man is preferable to her. Alternatively, because of "they were diligent." For it is taught: Why did they say a widow marries on the fifth day and is wed on the sixth? Because if you say she is wed on the fifth, the next day he rises early to his craft and goes off; the sages were diligent for the welfare of the daughters of Israel, that he be joyful with her three days, the fifth day, the eve of the Sabbath, and the Sabbath. What difference is there between "blessing" and "diligence"? Between them is the case where the man is idle [from work], or alternatively, a festival that falls on the eve of the Sabbath.
Bar Kappara on the Proper Wedding Days for Brides
Yalkut Shimoni on Torah 16:4
(בראשית א כח) וַיְבָרֶךְ אֹתָם אֱלֹהִים וַיֹּאמֶר לָהֶם אֱלֹהִים פְּרוּ וּרְבוּ, תָּנֵי בַּר קַפָּרָא, בְּתוּלָה נִשֵּׂאת בִּרְבִיעִי וְנִבְעֶלֶת בַּחֲמִישִׁי, הוֹאִיל וְנֶאֶמְרָה בּוֹ בְּרָכָה לַדָּגִים אַלְמָנָה נִשֵּׂאת בַּחֲמִישִׁי וְנִבְעֶלֶת בַּשִּׁשִּׁי, הוֹאִיל וְנֶאֶמְרָה בּוֹ בְּרָכָה לְאָדָם אַלְמָנָה נַמִּי תִּבָּעֵל בַּחֲמִישִׁי הוֹאִיל וְנֶאֱמְרָה בּוֹ בְּרָכָה לַדָּגִים, בְּרָכָה דְּאָדָם עֲדִיפָא לֵיהּ. אִי נַמִּי, מִשּׁוּם שָׁקְדוּ, דְּתַנְיָא, מִפְּנֵי מָה אָמְרוּ אַלְמָנָה נִשֵּׂאת בַּחֲמִישִׁי וְנִבְעֶלֶת בַּשִּׁשִּׁי, שֶׁאִם אַתָּה אוֹמֵר תִּבָּעֵל בַּחֲמִישִׁי לְמָחָר מַשְׁכִּים לְאֻמָּנוּתוֹ וְהוֹלֵךְ שָׁקְדוּ חֲכָמִים עַל תַּקָּנַת בְּנוֹת יִשְׂרָאֵל שֶׁיְּהֵא שָׂמֵחַ עִמָּהּ שְׁלֹשָׁה יָמִים, חֲמִישִׁי בְּשַׁבָּת, וְעֶרֶב שַׁבָּת וְשַׁבָּת. מַאי אִכָּא בֵּין בְּרָכָה לְשָׁקְדוּ אִכָּא בֵּינַיְהוּ אָדָם בָּטֵל אִי נַמִּי, יוֹם טוֹב שֶׁחָל לִהְיוֹת עֶרֶב שַׁבָּת.