R' Elchanan points out that there seems to be different views in Tos. with respect to the relationship between tosefes shabbos and the actual kedushas hayom. Tosfos (Kesubos 47) writes that the halacha of "ain me'arvin simcha b'simcha" does not apply during tosefes shabbos or yom tov and one can hold a wedding then. This seems to indicate that the kedushas hayom of shabbos/yom tov is not yet in effect during the time of tosefes. However, Tosfos elsewhere (Pesachim 99) quotes R"Y M'Korbil that if not for being barred by a special gezeiras ha'kasuv, one could theoretically fulfill the mitzvah of korban pesach, matzah, and maror during the time of tosefes. This seems to indicate that the full kedushas hayom of the upcoming day is in effect during the time of tosefes.
Rav Wahrman resolves this inconsistency by positing that there is a difference between halachos which define the kedushas hayom and halachos which are merely an outcome of there being a kedushas hayom. He offers the following example:
The gemara (Shabbos 69) says that it is possible to be chayav 39 different chata'os if one is aware that it is shabbos and violated each of the 39 different melachos b'shogeg. How is such a case possible -- if one doesn't know any of the 39 melachos are prohibited, in what sense can one be aware that it is Shabbos? The gemara answers (according to Reish Lakish) that one is aware that there is an issur of techumin.
Tosfos asks why the gemara did not answer that the case is where the individual is aware of the mitzvos aseh of shabbos. The Achronim go further: Why did the gemara not simply answer that the case is where the person made kiddush or knew there was a mitzvah of kiddush?
The answer, writes Rav Wahrman, is that melachos or techumin, are part of the definition of the kedushas hayom of shabbos. The mitzvah of kiddush is a result, an outcome of there being a kedushas hayom -- i.e. since it is shabbos, one must say kiddush, but the saying of kiddush does not define the day as shabbos.
"Ain m'arvin simcha b'simcha" is a result, an extension, of their being a kedushas hayom of shabbos or yom tov. The mitzvah of korban pesach, of matzah, or maror, are part of what define the kedushas hayom of pesach. (Seems to me that this is a very subtle distinction!)
(Rav Wahrman mentions that R' Hershel Shachter has a different approach to explaining why the gemara in Shabbos defines awareness of shabbos only by reference to the melachos but not the mitzvos aseh of shabbos. RHS suggests that the concept of kedusha is created specifically through added issurim. Kedushas kehunah (kedusha of people) is reflected in the fact that a kohen cannot marry a gerusha and other issurim. Mitzvos that are teluyos in kedushas ha'aretz (kedusha of place) like terumah create issurim -- tevel. Kedushas shabbos (kedusha of time) is reflected specifically in the awareness that shabbos carries with it varios prohibitions. This may explain why according to one view in Tosfos (Baba Basra 81) the mitzvah of bikurum is not a mitzvah ha'teluya ba'aretz -- since there is no issur of tevel created if bikurim are not separated, the mitzvah cannot be considered connected with kedushas ha'aretz.)