Easter eggs, also called Paschal eggs, are decorated eggs that are usually used as gifts on the occasion of Easter or springtime celebration. As such, Easter eggs are common during the season of Eastertide (Easter season). The oldest tradition is to use dyed and painted chicken eggs, but a modern custom is to substitute chocolate eggs wrapped in colourful foil, or plastic eggs filled with confectionery such as chocolate. Although eggs, in general, were a traditional symbol of fertility and rebirth, in Christianity, for the celebration of Eastertide, Easter eggs symbolize the empty tomb of Jesus, from which Jesus resurrected. In addition, one ancient tradition was the staining of Easter eggs with the colour red "in memory of the blood of Christ, shed as at that time of his crucifixion." This custom of the Easter egg can be traced to early Christians of Mesopotamia, and from there it spread into Russia and Siberia through the Orthodox Churches, and later into Europe through the Catholic and Protestant Churches. This Christian use of eggs may have been influenced by practices in "pre-dynastic period in Egypt, as well as amid the early cultures of Mesopotamia and Crete". 

The practice of decorating eggshells as part of spring rituals is ancient, with decorated, engraved ostrich eggs found in Africa which are 60,000 years old. In the pre-dynastic period of Egypt and the early cultures of Mesopotamia and Crete, eggs were associated with death and rebirth, as well as with kingship, with decorated ostrich eggs, and representations of ostrich eggs in gold and silver, were commonly placed in graves of the ancient Sumerians and Egyptians as early as 5,000 years ago. These cultural relationships may have influenced early Christian and Islamic cultures in those areas, as well as through mercantile, religious, and political links from those areas around the Mediterranean.
what season are they commonly used?
Easter eggs are common during the season of Eastertide (Easter season