Following קריאת
התורה on שבת
morning (in some congregations on weekday mornings as well) we recall the
souls of the departed. This is based on דברים כא:ט as darshened by ספרי there, as follows: כפר לעמך refers to the living;
אשר פדית refers to the
departed, meaning that the dead need כפרה. To this end, one whose relative’s yahrzeit or
the like was the past week makes a memorial prayer and pledges צדקה in order to atone for
the sins of the departed.
We do so specifically on שבת because at that point the shuls are full and those who are attending can take to heart that one day they will also need their sins atoned for, to which end they should be involved in תשובה ומעשים טובים while they still have the opportunity to do so.
Similar to אבהרחמים, which is a communal memorial prayer on behalf of martyrs collectively, individual memorial prayers (referred to in ספרדי tradition as השכבות, while having no formal title in the אשכנזי one) are subject to rules as to where they are said or not. While each congregation is entitled to establish its own custom in this regard, nevertheless we present here some broad guidelines as put forth by the פוסקים.
1) השכבות are not said:
a.
On שבת מברכים (even during ספירה, during which we
would say אב
הרחמים) including שבת
מברכים חודש אב and שבת
ר"ח אב
i.
Except for someone who was
buried that week (according to some opinions)
b.
שבת ר"ח אייר
c.
All of חודש ניסן (even what of it that
overlaps with ספירה,
where we would say אב
הרחמים)
2)השכבות are said on any other
occasion at which there is קריאת
התורה (I have seen those who are מקפיד not to say השכבות on ר"ח, though this is by no
means universal).
References: רמא או"ח רפד:ז, מ"א שם ס"ק ז-ח, מ"ב שם ס"ק יז-יח