I know that the universal ditress call of SOS was for the ease and simplicity of sending the call over the old wireless equipment. I’ve noticed (movies, tv, etc.) that when using a voice radio the term SOS is never used but instead the distess call of “Mayday” is used. Where did the term come from, what exactly does it mean, and when’s the appropriate use for either SOS or Mayday?
From what I learned for the marine VHF license MAYDAY was derived from m’aidez but it was was not an informal corruption but a deliberate invention of a word that was easily pronounceable by both English- and French speakers.
At sea MAYDAY (i.e. a distress call) is only to be used in emergencies when the ship itself or everyone on board is in danger.
The lesser degree of urgency is an urgent message, beginning with PAN PAN, to be used e.g. in case of Man Overboard or when a vessel needs assistance but is in no immediate danger.
Mayday and SOS are different calls for different degrees.
SECURITAY - Is a call transmitted in reference to safety: for instance: My boat has difficulty maneuvering
PAN: is a safety message of more urgent nature: my boat is blocking the channel
Mayday: is a call for help but not as urgent as SOS: my boat is taking on water and we need a tow but we are not in any immediate danger of sinking
SOS: is an urgent call for help: our boat is on fire and in immediate danger of sinking plus we have an injured crew and we can’t find the keys to the liquor cabinet.
Aviation doesn’t use SOS (so far as I know) over the radio. It does use PAN for “this is serious, but not immediately life-threatening” and MAYDAY for “if we don’t fix this we’re all gonna die!”
Similar responses in English speaking nations will be obtained by “URGENT” and “EMERGENCY”, but pan and mayday will work anywhere (at least in theory)
That explains why you don’t say Ascension Sunday 
The “SOS” distress call technically doesn’t isn’t composed of individual letters – it actually the Morse Code sequence …—… or three dots/three dashes/three dots all run together. However, in International Morse three dots are the letter S, and three dashes the letter O, so it is commonly written out as “SOS”. Germany adopted the SOS distress call in 1905, but there were at least two other competing distress signals at the time, CQD, used by the Marconi stations beginning in 1904, and NC, used by the U.S. Navy. At the 1906 International Radio Convention in Berlin it was decided to use SOS as the international standard, effective July 1, 1908, although it took a few years for the Marconi stations to make the switch.
SOS was adopted at a time when radio transmitters could only transmit in Morse code. As noted above, with the development of audio transmitters “mayday” was adopted as the audio distress call. However, its easier to stamp out “SOS” in the snow than “mayday”.
Incidentally, “SOS” doesn’t mean “save our ship” (or “save our souls”) according to what I have read. As whitetho says, the Morse code for SOS is three dots, three dashes, and three dots.
Emergency signals are often in threes. So why not have three sets of three dots? Because (I presume) “… — …” is more clearly three sets of three than “… … …”, which may sound likt “…” if someone is a little panicky.
We’ve all heard the “fire three shots in the air” thing if you’re lost in a forest. I don’t know if this is “official”, but enough people “know” about it that it might work.
Three fires placed in the form of an equilateral trianle is also a recognized distress signal. During the day, you might want to use panel markers.
I’ve bookmarked the army field manual for distress signals, but it’s on my office computer. If anyone is interested, I’ll try to remember to post the FM-number.
Another distress signal is to fly the US flag upside down. I guess if you don’t have a US flag any upside down flag will probably do the trick.
Good article on distress calls here.
MAYDAY and SOS are equal in distress urgency. The only difference is you never say SOS in a “phone” or voice transmission, SOS is for use only in Morse Code.
No, it won’t work with British, French or Japanese flags…
I have to disgree on the British flag. The Union Jack is not symmetrical, and anyway it’s not used as the national ensign on British ships - British merchant ships use the Red Ensign.