In agriculture, some crops reach their full potential during the days between Pesach and Shavuos. The crops are fully grown by Pesach time (האביב חג), and Shavuos time (הקציר חג) they are ready to be harvested. The Maharal (Chiddushei Agados, Rosh Hashanah) explains that this agricultural reality represents what is happening in a spiritual realm. On the days between Pesach and Shavuos, we improve until we reach the level of perfection by Shavuos.
On Pesach, we bring the korban Omer from barley, which is generally used as animal fodder. On Shavuos we bring the shtei halechem sacrifice, from wheat flour, which is fitting for human consumption. The meforshim (see, for example, Aruch HaShulchan 489:3) explain that this implies that we rise from the level of animals to the level of humans during these days. It is a time for growth.
The Ramban (Vayikra 23:36) says that the days of sefiras ha'omer have kedushah like Chol Hamoed. Just like Chol HaMoed comes between the first and final days of yom tov, sefiras ha'omer comes between Pesach and Shavuos.
This means that sefiras ha'omer is a holy and happy time. It is like Chol Hamoed. In contrast, sefiras ha'omer is also a time of mourning, as Reb Akiva's students were niftar then.
We don't make chasunos during the sefirah. We also don't perform weddings on Chol HaMoed. The Rizhiner explains that people who conduct themselves in an uplifted manner during the days of sefiras ha'omer, with happiness and holiness, as though these days are like Chol Hamoed, don't make chasunos during sefiras ha'omer because we don't make chasunos on Chol Hamoed. And people who don't have any spiritual joy on these days should refrain from making chasunos because these are days of mourning.
The Mishnah (Idiyos 2:10) states, "Five things were for twelve months: (1) The mabul (2) Iyov's afflictions. (3) The ten makos (4) The punishments for Gog and Magog (5) משפט חדש עשר שנים בגיהנם רשעים, the resha'im are punished in Gehinnom for twelve months... Reb Yochanan ben Nuri disagrees and says, העצרת ועד הפסח מן, the resha'im are punished in Gehinnom from Pesach until Shavuos..."
There are different explanations for the final words of this Mishnah. The Shibolei HaLeket (and other commentaries) hold that הפסח מן העצרת עד is literal. The resha'im are punished only between Pesach and Shavuos.
As we know, the purpose of Gehinnom isn't to punish. Gehinnom cleanses people of their aveiros so they can be worthy of going to Gan Eden. According to the Shibolei HaLeket, the purifying period of Gehinnom is during the days of sefiras ha'omer. That is the only time Gehinnom is active (according to this view).
The Chidushei HaRim adds that for us too, the purifying period is between Pesach and Shavuos, when we count the omer. If we take advantage of this time to purify ourselves, we won't need the purification process in the next world, Gehinnom ch’v.
