Sick on the Yahrzeit and unable to fast.

A person who has always fasted on the day of his father's death without stipulating that it is without a vow, and this year he is not feeling well. Does he need a release from the vow?

If he knew from the first year he began fasting that the fast is not an absolute obligation but rather a good practice, he needs a release from the vow with an opening. He should take three people, preferably one knowledgeable in the laws of vows, and tell them he wishes to annul his vow, and if he had known he would regret it, he would not have vowed, and they should tell him three times: "You are permitted."
If he cannot find someone to release him from the vow in a time of distress, he may be lenient and not fast even without a release from the vow.

The Shulchan Aruch, Yoreh De'ah, Siman 214 states: "One who is accustomed to fasting on the fasts before Rosh Hashanah and between Rosh Hashanah and Yom Kippur, and one who is accustomed not to eat meat and not to drink wine from Rosh Chodesh Av or from the 17th of Tammuz and wants to retract due to illness, needs a release from three people. If at the time he began observing, he intended to do so always, and observed even once, he needs a release."
The Shach there, S"K 2, explains that a release is needed because he did not consider that he would not fast if he were unwell, and therefore a release is needed. However, in Pitchei Teshuva there, S"K 1, it is written: "See in the Dagul MeRevavah, who wrote that the author refers to one who wants to completely retract and never return to his custom, therefore a release is needed, but if some ailment occurred and he wants to eat today and will return to his custom in the future, no release is needed." So too wrote the Magen Avraham in Siman 581.
In Shaar HaTzion, Siman 581, S"K 33, it is ruled: "It is possible that if he cannot find someone to release him, one can be lenient in a time of distress, as the Eliyah Rabbah and Derech HaChaim quoted the words of the Magen Avraham, and also the Dagul MeRevavah in Yoreh De'ah, Siman 214 supports the words of the Magen Avraham."
Moreover, if he performed a release of vows on the eve of Rosh Hashanah, he can rely on this release of vows in a time of distress for vows of mitzvah that he observed three times and did not say "without a vow."