OC 496: NO OBSERVANCE OF A SECOND DAY OF YOM TOV BY A FOREIGNER WHO CONVERTS IN ISRAEL
A person from abroad who converts in Israel and is still residing in Israel observes only one day of Yom Tov holiday instead of the two days observed by the Jews of the diaspora. This is true even if he will be returning to live in the Diaspora.
OC 529: THE OBLIGATION OF FELLOW JEWS TO HELP THE CONVERT REJOICE ON YOM TOV
Fellow Jews have a special obligation to feast with converts on Yom Tov since holidays are a time when a convert is likely to feel most alone.
OC 566: CONVERSION ON A FAST DAY
One who converts on a fast day should start fasting from the beginning, even before the conversion, so that one can fulfill the obligation to be fasting to the appropriate degree at the time that one emerges from the mikva. This suggests to some authorities that one who ate before conversion need not bother fasting. However, inasmuch as eating and drinking on a fast day are sinful in and of themselves each time one does so even if one has already eaten or drunk, some rule that even the convert who ate earlier must join the fast after conversion. In any case, wine clearly should not be used when the convert is named on a fast day – unless it will be given to someone who is permitted to drink wine that day.
CONVERSION ON A POSTPONED FAST DAY (TAANIT NIDCHE)
There is some discussion whether one who converts on a minor fast day that due to Shabbat was postponed from its official date (“taanit nidche”) is required to fast with everyone else. This is because one could argue that a person that wasn’t Jewish on the official date of the fast never became obligated to fast. However, even if we put the complicated discussion of the status of a taanit nidche aside as beyond the scope of this work, the mitzva of “you shall love the convert” suggests that a convert should fast with everyone else. Since allowing the convert to observe as all do is generally what the convert wishes, it manifests love.
CONVERSION ON SUNDAY THE 10TH AV (TISHA B’AV NIDCHE)
When one converts on Sunday the 10th of Av (a Sunday on which the fast of 9th Av is observed instead of on Shabbat), there is even more reason to fast. This is because that Sunday the 10th of Av more than being a mere fast postponed (from its original date on which the convert had not yet been Jewish and obligated) might have the status of an actual fast date inasmuch as the 10th of Av was one of the days during which the Temple had been destroyed (and so the convert is undoubtedly obligated).