In the legal code of Justinian, issued in 534 AD, we find the following entry, in book 3, title 12, law 6, Omnes dies:
3.12.6 (7). Emperors Valentinian, Theodosius and Arcadius to Albinus, City Prefect. We order that all days shall be court days.
1.Only those days shall remain as days of vacation which each year, for a period of two months, indulgently gives to rest from labor, in order to mitigate the summer heat, and to gather the autumnal fruits.
2. We also give over to leisure the usual days at the beginning of each year.
3. We add to these the natal days of the great cities of Rome and Constantinople, during which the legal proceedings, which own their origin to them, should be deferred. We also add the holy pascal days, seven preceding and seven succeeding Easter, also the natal day of Christ and the day of Epiphany and the time during which the suffering of the apostles, the teachers of all Christianity, is rightly commemorated; and these holy days shall not be open to shows.
4. Sundays, too, which the ancients rightly named the Lord’s days and which return at regular intervals, shall be put in this class.
5. An equal reverence shall be paid to the days which marked our birth and the beginning of our reign, and on these days no examination of disputes shall be made before referees, whether appointed by judges upon request or by the choice of the parties.
6. During the fifteen days of Easter, the exaction of all taxes in kind and of all public and private debts shall be deferred.
Given at Rome August 8 (389). C. Th. 2.8.2.[1]
The law is copied from the Theodosian Code, and is apparently enacted in 389 under Valentinian. Note that it is ordered that the Christian “holy days” do not have the games celebrated on them.
But comparison with the text of the Codex Theodosianus[2] shows that in fact Justinian, on reissuing it in 534 AD, has added some holidays; Christmas, Epiphany, and the festivals in commemoration of the apostles. Sadly I don’t have the English translation in PDF, but the Latin can be followed easily enough.
CTh.2.8.19 [=brev.2.8.2]
Imppp. Valentinianus, Theodosius et Arcadius aaa. Albino pf. U. Omnes dies iubemus esse iuridicos. Illos tantum manere feriarum dies fas erit, quos geminis mensibus ad requiem laboris indulgentior annus accepit, aestivis fervoribus mitigandis et autumnis foetibus decerpendis.
1. Kalendarum quoque ianuariarum consuetos dies otio mancipamus.
2. His adiicimus natalitios dies urbium maximarum, Romae atque Constantinopolis, quibus debent iura deferre, quia et ab ipsis nata sunt.
3. Sacros quoque paschae dies, qui septeno vel praecedunt numero vel sequuntur, in eadem observatione numeramus, nec non et dies solis, qui repetito in se calculo revolvuntur.
4. Parem necesse est haberi reverentiam nostris etiam diebus, qui vel lucis auspicia vel ortus imperii protulerunt. Dat. vii. id. aug. Roma, Timasio et Promoto coss.
While the sacred days of Easter (Sacros paschae dies) plus seven days before and after are part of the Codex Theodosianus, the words “also the natal day of Christ” onwards were added later than the Theodosian code, itself published in 438 AD.
This interesting snippet came to me via a very old volume, E. V. Neale’s Feasts and Fasts from 1845, p.23.[3]
So it looks as if we may reasonably attribute the creation of Christmas as a public holiday, rather than merely a church festival, to Justinian.