There are various customs and even halachot associated with yahrtzeits, such as saying kaddish, getting called to Torah, lighting candles, fasting, serving as a prayer leader, giving charity, arranging a minyan at the cemetery, etc. What is the earliest or most authoritative source for any of these customs/halachot?
Nitei Gavriel Aveilus2 75:1:1 brings the following sources for Yarzheit candles.
Minhagei Vermaisha Volume 2, Page 112
Maharshal Chapter 46
Magen Avraham 261:6
Nachalas Shiva Chapter 73
Kitzur Shulchan Aruch 221:1
Mishmeres Shalom Note 10:10
Gesher HaChaim 32:4
Will try to add more customs and sources later.