The conflict for St Kateri may be because different countries have it on different days, or because when her normal feast of June 14 falls on Sunday, certain parishes named after her and her local diocese still wish to celebrate her day, but cannot celebrate her feast on Sunday so they move it to the 15th.
For saints who predate Vatican II, many of them had a feast day on the 1962 Tridentine Calendar that moved to another day when the calendar was revised after VII, or their feast was dropped. Also, the calendar went through a lot of revisions before 1962 and occasionally a saint’s day was moved. The 1962 and earlier calendars don’t apply to St Kateri because she was canonized after Vatican II.
Finally, be aware that some Eastern saints are celebrated on one day by the Eastern Catholics and on another day by the Roman Catholics.
Once in a while, a feast day that you see online is also just wrong because someone made a mistake.
I find Wikipedia to be a pretty good reference for saints’ feast days.