The grace of Ordination is very special and very wonderful, so the Orthodox have always been motivated to restrict ordinations so that they cannot become common, too easy, or too commonplace.
In the Eastern rite, this is done by having only one ordination to one rank on one day.
In the Western rite, this is done in rather an opposite fashion, by restricting ordinations to the four Ember Saturdays of the year (an ancient fast observance even older than Lent itself), on which days all the ranks and orders and their ordinations are performed "en masse," with great fasting and reverence and maximum lengthiness of divine services.
The two seem very different on the surface, but arise from a common understanding and a common impulse.
I hope this is helpful.