EXPLANATION When letters are pronounced with a hissing sound (as in s, sh, ch, z), we call them sibilants. The sibilant letters in these words have been highlighted:
Francis
Alexis
signal
spend
Boadicea
Tarsus
Xerxes
Philippoussis
Sanchez
Jesus
axis
Sussex
With normal singular words (words indicating a single thing), we indicate possession by adding 's (for example, the boy's skateboard). Many grammar books suggest that if the last syllable of a word begins and ends with a sibilant sound, as in Francis, we should simply add an apostrophe to show possession. The reason for this exception is that, if we were to follow the normal rule, we would end up with Francis's. In other words, there would be three sibilants close together and some people feel this sounds a little clumsy when we pronounce it.
Pronunciation is not a particular problem in writing, of course, but some people still prefer to write Francis' instead of Francis's. It is not wrong to write Francis's. The main thing is to choose a style and then be consistent. In fact, if you find apostrophes confusing, it is probably best just to add 's to all singular words to indicate possession - even if they are sibilant words.
Note that some grammar books provide other variations. They suggest that if a long noun ends in an s or a z, only an apostrophe need be added to show possession - even if the last syllable does not begin with a sibilant sound. Using this rule, we would write Maria Stanopoulos' wedding rather than Maria Stanopoulos's wedding. Again you might find it less confusing if you ignore exceptions and just add 's to all singular words (and so write Maria Stanopoulos's wedding, for example).
The main thing about styles is to choose one and be consistent.