Colin wrote:Spirithealer and Frank wrote:Kyō dake ha (Today only)
I've not come across that version before!
I use Kyō dake
wa
I already expected such a remark

The reason why you never came across that 'version' before is because most people don't understand Japanese.
Or because people just copy what they learned.
I don't want to say I know a lot about it (hell no, I know very little), but I have tried to come up with my own translation of the Gainen/Gokai. I tried to understand some of the grammatical structures. There is a lot about Japanese that I don't understand, but I do understand some aspects.
The Japanese use kanji or kana (hiragana/katakana). There is also something called rōmaji; rōmaji is used when people want to transliterate kanji/kana into our Western script based on the Latin alphabet.
"Today only" in kanji and kana looks like this: 今日丈けは
(Hope you have installed kanji/kana on your computer)
In rōmaji it is: kyō dake ha
Why do I write "ha" in stead of "wa" ?
Because the character used for "ha" is a particle (it has a grammatical function - states what the subject is you talk about) and in rōmaji it is spelled "ha".
However, the pronunciation of the particle is "wa".
So, it should be spelled "ha" and it should be pronounced "wa".
Something similar happens with "Gō wo hageme".
"Wo" should be spelled "wo" in rōmaji. It is, however, pronounced "o".
Sometimes pronunciation is different from the way one should write a word.
I hope this helps

*Gasshō