For the source text click/tap here: Yevamot 43
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In the Mishnah (41a), Rebbi Yosi permits a woman (who was divorced) to do Erusin with a second husband immediately without having to wait three months. The only exception is a woman whose husband died, in which case the woman must wait before she remarries, not because of the requirement of "Havchanah" but because of her Aveilus.
Our Daf distinguishes between avelut hadashah and avelut yeshanah, "new" mourning and "old," historical mourning - or, expressing the same thought in a different idiom,
between avelut de-yahid and avelut de-rabbim, private and national-communal mourning.
The first, avelut hadashah, is caused by a death or disaster which strikes a family or an individual. It is a primordial, instinctual, spontaneous response of man to evil, to the traumatic confrontation with death, to the impact of catastrophe and disaster.
The second category, avelut yeshanah, is due to a historic disaster that took place 1,900 years ago. This category is the handiwork of man. There is no spontaneous reaction to some new event which has just transpired, for nothing new has happened which should justify grief.