Going to school either for the first time, or moving up or the last year before the 'big wide world' beckons.

Its also a day of possible trauma for the young children and the not sufficiently sensitive psychological issue of 'separation anxiety' by this I mean what the great Dr John Bowlby in his landmark book on Attachment. This seminal work became the touchstone of many in the helping profession and should be top of any reading list. It forms the basic knowledge of our understanding of the intra-psychic world of the infant and young child as their inter-personal world is changed around them. For those who have the best interests and welfare of the long term mental welfare of children at heart and their society this must be read and acted upon. Regrettably there are too many 'social policies' that lack sufficient attention to this necessary human condition of attachment, separation, anxiety, loss and depression. There are few works that change the world as we see it, this is just such a work and describes so eloquently and more important empirically the cardinal and central role of a consistent parental person or their substitute'. We now know in a way that was not sufficiently understood in the past the central concept of consistency at the very young age of a familiar person who may be or act in loco parentis. 
When we say goodbye to the young child for the first time this is an 'indelible event for them and for us'. Moreover when they graduate from nursery as many will today to take their first steps on their long journey to adulthood and maturity these very early experiences and how they have been dealt with will hang over them like a 'shadow all their adult lives' for good or ill.
It cannot be overstated too strongly just how important this 'milestone' is for later adult mental health and how their families have or have not negotiated them. Too often its disregarded as it is seen as an everyday occurrence, but the thoughtful parent or their substitute will know its the foundation of life itself and later self esteem and the ultimate ability as an adult to fulfillment and happiness and successful relationships and children. So an important day for everyone.