In Roman times, this was part of series of celebrations related to Cybele and Attis, representing a cycle of death and rebirth.
The seven following days the Canna Intrat (15th March) were famous as Castus Matris ("Fast of the Mother"). On March 22nd the procession of the Arbor Intrat took place. Before sunrise, a pine tree was felled in a sacred grove of Cybele. An effigy of Attis was attached to it and decorated in violets and ribbons. It was then carried in procession through Rome by the dendophori (to the weeping of the gallae) to the sanctuary, where it was exposed to the adoration of the crowd before being laid in state.
The following day was a "day or mourning" and lamentation. The Salli (who were priest dancers of Mars) went in procession sounding their trumpets and beating their shields.
In our life we undergo many transformations and transitions, not just ones that we focus on. They are not always wanted and often we defer them (if we can) through fear and insecurity. Sometimes we embrace and initiate these changes, making them the centres of our lives for better or worse. And with change (even change we desire) there often comes grief and mourning. Grief can come over lost relationships, or expectations and ideas of ourselves that have proven either false or unbearable.
One can deny such grief, pretend it's not there or that it doesn't matter, but by doing so we also depreciate the worth of what we grieve for, and deny our history. Rather, it is often better to mourn for things lost, because this is ourselves owning our pain. In doing so we can acknowledge the role that such relationships and expectations played in our lives, and in doing so can also accept the good that came from them.
Mourning takes - however long it takes. And when mourning finishes, if we've paid attention to ourselves, often we can be refreshed and ready to move on.