We read in Parashat Pekudeh (40:2) of G-d's command to Moshe that the completed Mishkan should be erected on Rosh Hodesh Nissan, the first day of the month of Nissan.
The Midrash tells that in truth, the artisans built the various parts of the Mishkan very quickly, and the Mishkan was ready to be assembled already earlier, on the 25th of Kislev, a little over two months after the work began. However, Hashem decided to postpone the assembly of the Mishkan until the month of Nissan, due to the unique significance of this month. The Midrash continues that the 25th of Kislev "protested," as it were, objecting to its having been denied the privilege of being a special day, the day of the Mishkan's inauguration. Hashem assured this day that it will be compensated – and, sure enough, many centuries later, the 25th of Kislev became the first day of the joyous holiday of Hanukah, celebrating the Jews' miraculous triumph over the Greeks and the rededication of the Bet Ha'mikdash, which the Greeks had defiled. The day of 25 Kislev was at first denied its holiday – but it was later compensated with "interest." Whereas the first of Nissan, commemorating the inauguration of the temporary Mishkan, is not marked by a great deal of fanfare, the 25th of Kislev became a day of great festivity observed by Jews around the world for millennia.
The Midrash's comments bring to mind a different Midrashic account, regarding the fish in the Yam Suf (Sea of Reeds). When Hashem drowned the 600 Egyptian horsemen who had pursued Beneh Yisrael into the sea, the fish were overjoyed, and prepared to indulge in the robust feast that had just been presented to them. But just as they were set to eat, the bodies of the Egyptian horsemen were sent ashore. Beneh Yisrael feared that the soldiers had not drowned, but rather came ashore, and thus still posed a threat. In order to allay the people's concerns, Hashem had the sea expel the remains of the Egyptian warriors onto the shore where they were standing, so they would see them and know that their pursuers were indeed dead, and they were thus no longer in danger. The fish were, naturally, disappointed over the sudden loss of the large amounts of food that they thought they would soon be enjoying. Hashem repaid the fish years later, during the battle waged by Beneh Yisrael in the times of the prophetess Deborah against the army of Sisera. During this war, Hashem drowned the enemy's 900 chariots in the Kishon River. As compensation for having lost the 600 Egyptian horsemen – the fish were given 900 Canaanite soldiers.
This is a reason given for the custom that many have to ensure to eat fish on Shabbat. Many people might have serious misgivings about closing their businesses on Shabbat, or about taking off from work, worrying about the loss of income. The fish reminds them that Hashem always repays in full – and with "interest" – for the sacrifices we make for Misvot. Whatever expense or loss we incur for the sake of a Misva is more than worth it – because we are guaranteed to receive much more than we've lost.
The Gemara (Besa 15b) teaches that Hashem tells us: "My children! Borrow on My account to make the day [of Shabbat] sacred, and trust Me that I will repay." Hashem guarantees that all the money we spend for Shabbat and holidays is being charged to His account, and He will repay us in full. And, as we have seen, He not only repays the money we spend, but also gives us much more.
Performing Misvot is always the greatest investment we can ever make, one which guarantees to bring us the most profitable returns.