Shavuos - Omer
The Torah is going through various details of the various holidays. It discusses the Omer offerings and upon arriving at the end of the Omer count (i.e. Shavuos), one has to bring a sacrifice called "shtei ha'lechem" - the two loaves. In 23:17 the Torah tells us, "מִמּוֹשְׁבֹתֵיכֶם תָּבִיאּוּ לֶחֶם תְּנוּפָה, שְׁתַּיִם שְׁנֵי עֶשְׂרֹנִים--סֹלֶת תִּהְיֶינָה, חָמֵץ תֵּאָפֶינָה: בִּכּוּרִים, לַיהוָה" or in English, "Ye shall bring out of your dwellings two wave-loaves of two tenth parts of an ephah; they shall be of fine flour, they shall be baked with leaven, for first-fruits unto the LORD."
This korban had a very unusual detail that makes it practically unique. It was made from chametz - leaven. It is one of two korbanos that contain leaven (the second is the korban Toda). All other sacrifices, including all the various mincha offerings, including the lechem ha'panim, including all other dough that would be offered in avrious forms in the mikdash, were leaven free. Kosher for Passover. Matza, albeit it thick and soft usually.
What is the significance of this sacrifice, the "Two Loaves" being made from chametz, unlike almost every other korban?
This offering, the שתי הלחם, is brought at the conclusion of the omer. The Omer count began on Pesach and concludes on Shavuos. In a sense, the Omer is the bridge between Pesach and Shavuos.
Pesach is יציאת מצרים - the Exodus from Egypt. Shavuos is מתן תורה - the giving of the Torah. Omer, the bridge between them, is the process of leaving Egypt and working towards מתן תורה. It is the time to prepare for the receiving of the Torah.
On Pesach we ate לחם עוני - poor man's bread, a.k.a. the Bread of Affliction. We eat the matza to remember the days of slavery and oppression. By doing so we recall the process of leaving Egypt and leaving that oppression behind. Shavuos is when we finally hit the target and got the Torah. That is when we ultimately became free and a nation.
To symbolize this we offer the rare sacrifice with חמץ - the opposite of poor man's bread. This is the bread of the free and the wealthy. The un-oppressed. Rich man's bread, if you must. When we get to Shavuos we can finally say the עוני of מצרים is behind us.
We start with poor man's bread, we work towards our freedom and we end with rich man's bread.