Christ is Risen!
Akathist literally means "while standing" in Greek. It has a connection with the original form of the Kontakion, which was written as an acrostic of the Greek alphabet and contained the proeimeion and the same number of oikoi as letters of the Greek alphabet.
The first known Akathist was to the Theotokos and is attributed to St. Romanos the Melodist. The Akathist by tradition was first used publically in Constantinople in 626 in thanksgiving for victory over an attack of the Persians.
Later the kontakion was shortened to the version you see used in Byzantine services, but the Akathist to the Theotokos remains in this original form as a "proto-kontakion". It probably remained in this form because it was also a very popular paraliturgical service which could be sung inside or outside of a church and was used in all kinds of natural disasters, wars, pestilence, etc. and was also used in Great Lent.
In Greek practice the Akathist to the Theotokos is still sung along with the Canon to the Theotokos of Joseph on Fridays of Great Lent at Compline, broken into portions each of the first four weeks and sung in its entirety on the fifth Friday. In Slavic practice the Akathist to the Theotokos is sung on Akathistos Saturday (fifth Saturday of Great Lent) alone.
Besides these "official" days when it is prescribed to be sung, it is often sung after Liturgy or separately at other times as an intercessory prayer.
Akathists to Christ, His Passion, Resurrection, the Holy Cross, to various saints, and for specific feast days have followed the original in terms of general structure, but some variations exist, such as the number of "Rejoices" (chaires). In current usage, the Akathist repeats the last Ikos (13) three times, and then Ikos 1 is repeated followed by Kontakion 1, often with one or more concluding prayers at the end.
In the Ukrainian, Belarussian and Russian traditions, there are hundreds of different Akathists in existence.
FDD