The word Sesshin is used in Zen cireles for a period of time in which the
practice of Zazen (sitting meditation) is conducted as the main occupation of the participants.
Sesshin can last something between 1 and 10 days, and the hours of Zazen
practice during a Sesshin day can vary between 8 and 16 hours.

The word Sesshin literally means:
Se = to touch, to get in close contact;
Shin = our heart-spirit (our original, true being).
So Sesshin means to make
the utmost effort to get in close contact, to become aware of our innermost
true being. This true being, our heart spirit, is always within us. From there
we are alive, from there our heart is beating and our liver is functioning.
And from there our senses perceive, and from there our compassion arises.
These vital abilities we usually take so much for granted, that we ignore
them. We are much more interested in what we perceive and whether we
like it or not. And the daily battle we fight to get what we like, and to avoid
what we don’t like lets us forget about our most inner being. When we are
able to return to this inner home, all struggles are forgotten. And we will be
able to enter again daily life – renewed, refreshed and with a sense of
unshakable trust in our self, that is to say, in our true being, that has
nothing to do with what we have learned at school and during our
professional career,

To overcome our intense fascination for the play with our likes and dislikes
strict discipline is asked for, Therefore Sesshin is always strict and
demanding in discipline. Zazen practice is nothing else than the effort
needed to get away from our entangled mind, which circles around the
qualities of the circumstances we live in, and which tries to get control of
them to our advantage, again and again. When we return to our true being,
we learn to accept things and circumstances as they are. We learn not to
fight against what is, but to live in harmony with the universe and
ourselves. Zazen means to ignore our thoughts and follow our breath deep
into our unconscious. Down there, well protected from our attempt to
consciously control everything, lies this treasure, the well, from which our lives
spring. In itself it is everlasting, serene, and seemingly empty – but full
with potential