Where does the concept of Oneg Shabbat come?
Rav Naḥman bar Yitzḥak said: One who delights in Shabbat is rescued from the oppression of exile. He derives it by means of a verbal analogy. It is written here, with regard to Shabbat: “And I will cause you to ride on the heights [bamotei] of the world” (Isaiah 58:14), and it is written there: “You are fortunate Israel, who is like you? A nation redeemed by God, the shield that aids you and the sword of your triumph. Your enemies will try to defeat you and you will trample their high places [bamoteimo]” (Deuteronomy 33:29). Rav Yehuda said that Rav said: With regard to anyone who delights in the Shabbat, God grants him his heart’s desires, as it is stated: “And you shall delight in God and He will grant you your heart’s desires” (Psalms 37:4). This delight in God, which is mentioned in the verse, I do not know what it is. When it says: “And you shall call the Shabbat delight,” one must say: It is the delight of Shabbat. The Gemara asks: With what does one delight in the day of Shabbat? Rav Yehuda, son of Rav Shmuel bar Sheilat, said in the name of Rav: With a dish of beets, and large fish, and heads of garlic. Rav Ḥiyya bar Ashi said that Rav said: Even with regard to a small item and one prepared it in deference to Shabbat, it is a delight. The Gemara asks: What is the small item mentioned? Rav Pappa said: Small fried fish.
How does the gemara expound upon the concept put forth in Isaiah above? Why do you think this reward is for oneg Shabbat specifically?
משנכנס אב ממעטין בשמחה כו' אמר רב יהודה בריה דרב שמואל בר שילת משמיה דרב כשם שמשנכנס אב ממעטין בשמחה כך משנכנס אדר מרבין בשמחה
§ The mishna teaches that from when the month of Av begins, one decreases acts of rejoicing. Rav Yehuda, son of Rav Shmuel bar Sheilat, said in the name of Rav: Just as when Av begins one decreases rejoicing, so too when the month of Adar begins, one increases rejoicing.
What are the events connected to the months of Av and Adar that require us to increase or decrease in joy?
יִשמְחוּ בְמַלְכוּתְךָ שׁומְרֵי שַׁבָּת וְקורְאֵי ענֶג. עַם מְקַדְּשֵׁי שְׁבִיעִי. כֻּלָּם יִשבְּעוּ וְיִתְעַנְּגוּ מִטּוּבֶךָ. וּבַשְּׁבִיעִי רָצִיתָ בּו וְקִדַּשְׁתּו חֶמְדַּת יָמִים אותו קָרָאתָ. זֵכֶר לְמַעֲשה בְרֵאשִׁית:
They shall rejoice in your kingship-those who observe the Sabbath and call it a delight. The people that sanctifies the Seventh-they will all be satisfied and delighted from Your goodness, and the Seventh-You found favor in it and sanctified it. 'Most coveted of days,' You called it, a remembrance of creation.
Tachanun is a set of supplications said on most weekdays, dealing with the theme of admittance of sins and repentance
And the widespread custom is to not say Tachanun for all of the month of Nissan, and not on Tisha B'Av, and not between Yom Kippur and Sukkot. [And not from the beginning of the beginning of the month of Sivan through Shavuot]
What is the significance of these days?
And the widespread custom is to not say Tachanun all the month of Nissan. Because on the first of Nissan, the tabernacle was built. And the twelve leaders of each tribe brought their sacrifices for twelve days. There was a day for each tribe, and each tribe made their day a holiday. And so too in the future, when the Holy Temple is rebuilt it will be rebuilt in Nissan.
Does this connect to what Rashi states above?