I have a Yartzeit on Motzei Shabbos-Sunday. How do I go about accepting it at Mincha Shabbos afternoon? Is it a problem of Hachana? Where can I find this written?
You are allowed to be mekabel taanis on shabbos, however you should not say the Yehei Ratzon part, which is a tefilah. As a side point If you usually fast on the day of the yahrtzeit even if you were not mekabel taanis it is as if it already done.
Rivivos Efrayim 269(2), Emek Halacha 36. Gesher Hachayim 32-6.