Assuming you aren't celebrating the newly created solemnity of (this year) Eleventh Day (aka 'Epiphany Sunday') you are probably celebrating the feast of the Most Holy Name of Jesus, popularly represented in the Christogram IHS (if you are facing an Epiphany Sunday, you probably celebrated the novus ordo version of the feast of the Holy Name on Friday).
But are the origins of the abbreviation?
I have to say I had assumed it was Latin, but at Mass this morning our priest suggested it was from the Greek.
A little digging suggests that the case can be made for either language.
The 1919 Catholic Encyclopedia gives it a Latin origin, saying:
"The emblem or monogram representing the Holy Name of Jesus consists of the three letters: IHS. In the Middle Ages the Name of Jesus was written: IHESUS; the monogram contains the first and last letter of the Holy Name. It is first found on a gold coin of the eight century: DN IHS CHS REX REGNANTIUM (The Lord Jesus Christ, King of Kings)."
This sounds very plausible to me, particularly as the abbreviation was popularised in the West in the fourteenth century (its first known appearance in written text is in the English Vision of Piers the Plowman).
The Encyclopedia entry goes on to explain some 'bacronyms' propagated by the Jesuits (who else!) and others:
"Some erroneously say that the three letters are the initials of: "Jesus Hominum Salvator" (Jesus Saviour of Men). The Jesuits made this monogram the emblem of their Society, adding a cross over the H and three nails under it. Consequently a new explanation of the emblem was invented, pretending that the nails originally were a "V", and that the monogram stands for "In Hoc Signo Vinces" (In This Sign you shall Conquer), the words which, according to a legendary account, Constantine saw in the heavens under the Sign of the Cross before the battle at the Milvian bridge (312)."
Nonetheless, some modern sources do propose Greek origins for the letters.
The Greek explanation
In particular, the Wikipedia, citing three recent authors, claims that:
"In Eastern Christianity, the most widely used Christogram is a four-letter abbreviation, ΙϹΧϹ — a traditional abbreviation of the Greek words for "Jesus Christ" (i.e., the first and last letters of each of the words ΙΗΣΟΥΣ ΧΡΙΣΤΟΣ — written "ΙΗϹΟΥϹ ΧΡΙϹΤΟϹ" with the lunate sigma "Ϲ" common in medieval Greek). On icons, this Christogram may be split: "ΙϹ" on the left of the image and "ΧϹ" on the right, most often with a bar above the letters (see titlos), indicating that it is a sacred name. It is sometimes rendered as "ΙϹΧϹ ΝΙΚΑ", meaning "Jesus Christ Conquers." "ΙϹΧϹ" may also be seen inscribed on the Ichthys. In the traditional icon of Christ Pantokrator, Christ's right hand is shown in a pose that represents the letters ΙϹ, Χ, and Ϲ....In the Latin-speaking Christianity of medieval Western Europe (and so among Catholics and many Protestants today), the most common Christogram became "IHS" or "IHC", denoting the first three letters of the Greek name of Jesus, iota-eta-sigma, or ΙΗΣ...Because the Latin-alphabet letters I and J were not systematically distinguished until the 17th century, "JHS" and "JHC" are equivalent to "IHS" and "IHC"."