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)