It is quite often one will see Roman numerals (I, II, III, etc.) in place of Arabic numerals (1, 2, 3, etc.) in my postings, often for numbered Sundays of seasons, and even (stealing my pastor's cue) for the month when writing a date (for example, today, as I write this, is July 8, 2015, or VII-8-15).
I know I'm probably preaching to the choir here, but I find it amazing that far less people are familiar with Roman numerals today than back when I learned them (first grade, that is, somewhere between September, 1970, and June, 1971). And it was first grade where I also learned that the numbers as we use them today (1, 2, 3, etc.) are Arabic numerals. (Side note: I also find it amazing that many younger people who work with money for a living, whether it be as a store cashier, bank teller, or even restaurant jobs, are unfamiliar with the terms "fin", "sawbuck", and "two bits".)
Anyhoo, I'm going to combine the two - Roman numerals and money! Using Roman numerals is very much like counting money. In United States currency, you have coins and bills (notes) of different value denominations. Roman numerals is the same thing. Observe:
I = 1
V = 5
X = 10
L = 50
C = 100
D = 500
M = 1000
When you have three one-dollar bills in your hand, you have three dollars ($3). Now, let's put three I's together - III - that is three ones (3). Now, if you put a sawbuck ($10 bill) and a fin ($5 bill) together, you have $15. In Roman numerals, you put the X and the V together - XV - and you get the number 15. For 125, you would use a C, two X's, and a V - CXXV. (Yes, 20 is two X's, there is no special Roman numeral for 20 - it's just XX.)
Roman numerals are basically written in the same order as Arabic numerals, left to right from highest "digit" to lowest "digit". However, you might see a case where one (and only one) lower "digit" might precede a higher digit. In that case, you would subtract. For example, IV is "one from five", which is four. XL is "ten from fifty", or forty. Twenty-nine would be rendered XXIX (two tens, plus "one from ten").
One note when subtracting, well, ok, two notes:
1) You can only subtract ONE lower digit from its higher digit. IIV is not 3, nor is XXC 80.
2) You can only subtract from the next five or ten up. In other words, 99 cannot be rendered IC. It would be rendered XCIX (10 from 100, plus 1 from 10). 95 would not be VC, but XCV. (IV = 4, IX = 9, XL = 40, XC = 90).
In numbering the Sundays, markings such as "II Advent" and "IV Lent" would be the Second Sunday of Advent and the Fourth Sunday of Lent, respectively. Anything marked "Sunday (numeral)" would be a numbered Sunday of Ordinary Time. For example, "Sunday X", "Sunday XXIII", "Sunday VI" would mark the Tenth, Twenty-Third, and Sixth Sundays of Ordinary Time, respectively.
Here endeth my basic lesson on Roman numerals.
PS: The other term I didn't cover: "Two bits" is a quarter (25 cents). ;)