The seven weeks of the Omer, between Pesach and Shavuot, are traditionally (though somewhat mysteriously) a time of partial mourning. Rabbi Shai Held offers this explanation:

The Exodus may be a paradigm for how Jewish history is supposed (indeed, destined) to look, but for now — tragically, inexplicably — history makes a mockery of this paradigm. Rome is triumphant; a renewed Exodus remains but a dim hope.

And so we mourn. We mourn because our experience falls so unbearably short of the redemption we have been promised and assured will come. There is a stunning degree of audacity — and honesty — in starting to grieve as Pesach begins, because, in fundamental respects, Pesach resides in the future rather than the present. And yet grief does not have the final — or even the loudest — word, because we affirm that the God who redeemed us will, despite all evidence to the contrary, redeem us “a second time.” (In The Heart of Torah, Volume 2, p. 75)

haggadah Section: Nirtzah