Drws rhwyng y blynyddoedd - "Doorway betwixt the years" (Boringly going with the Janus theme.)
Y mis wedyn y digrif un(/yn?) - "The month after the funny one" (DaysInMonth-wise.)
Hanner diwrnod am ynfydrwydd - "Half [a] day of fools" (Quixotic reference to April 1st, AM only.)
Y saeson dim ond defnyddio tri llythyren - "The English only use three characters [for this month]" (Also applies to many other languages, including the current Welsh one, I know.).
You'll doubtless be able to point out my "gwas ysgol" errors. As I said, I stuck to (AFAIK) unmutated forms of the words[1], and more or less retained the original English grammatical order. But with my sadly underused Welsh-English dictionary at home I was forced to improvise around a few words I half remembered, and some that I quickly looked up to make sure I wasn't using a wrong form. (i.e. going for "day, period", rather than "day, sunlit time".)
I know my road-sign Cymraeg much better than the spoken language. To within a llath or so, anyway.
Although I do rather pride myself on saying everything from Llangollen to Ystredfelle to Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch[2] better than your average visitor (although I forget whether I've got more of a northern or southern accent, when I do), I also like pronouncing one place as "[HL]andrindod We[hl]s" with an identical aspirant, to make fun of the double standards in its name.
[1] Did I forget to change "Y" to "Yr" or "'r", anywhere? That's usually the only thing I'm sure about. Except now I'm not even sure I remember
that correctly. Reading's easier. On the assumption that I'm reading something that's correct (and limited to just the more penetrable mutations) and not some idiot-like-me's bad rendition, of course...
[2] When I don't accidentally write "pwch" instead of "pwll" and "tislio" instead of "tyslio". Strange why I do that, because I know the differences. Probably left a typo in this post's version, anyway, knowing my luck.
(During the original writing of the above reply, my browser crashed. I think I upset my spill-chucker.)