If someone passed was buried erev chag Shavuout, I understand that the spouse mourning ends before 30 calendar days (shiva 1 day=7 days + chag = 7 days, + 2nd day chag = 1 day, leaving 15 days to reach sheloshim (mid June). However, I’m also being told that is only for the formal end of aveluit, but for making a misibah, Hazkarah, kaddish, visit to the gravesite, should not occur until 30 calendar days (near end of June).
The end of the shloshim is on 22 of Sivan, (this year it is on June 14th),
This that a Yom Tov ends shiva or shloshim is said in regard the halachos of mourning (showering, haircuts, etc) that are incumbent on the mourner. Kaddish, davening as the chazzan, which are done for the benefit of the deceased, are counted in the regular fashion, until 30 days from the burial. Regarding visiting the grave, hazkara etc., there is controversy about this, and a number of Poskim say that it should be done at the end of 30 days. If it is difficult to make it at the end of 30 days for whatever reason, it can also be made on the day of the end of the aveilus of shloshim, which is in line with the other Poskim.
M:B 548-52 in the name of Pri Megadim, Divrei Sofrim 51-68 and footnote quoting R’ S. Z. Auerbach zt”l, which is not like the opinion of Gesher Hachayim.