Eikev is one of the sections of Moshe’s farewell speech. It opens וְהָיָה עֵקֶב תִּשְׁמְעוּן, אֵת הַמִּשְׁפָּטִים הָאֵלֶּה, וּשְׁמַרְתֶּם וַעֲשִׂיתֶם, אֹתָם-וְשָׁמַר ה אֱלֹהֶיךָ לְךָ, אֶת-הַבְּרִית וְאֶת-הַחֶסֶד, אֲשֶׁר נִשְׁבַּע, לַאֲבֹתֶיךָ – When you will listen to the laws; safeguard and perform them, Hashem will safeguard you, and uphold the covenant sworn to your fathers (7:12).

The Alshich points out the peculiarity of the word עֵקֶב – it is very infrequently found; in addition to which, it doesn’t even seem to change the basic message of “When you listen”, were it not to say עֵקֶב.

Further, it seems to be a conditional term, but this raises an issue – in Judaism, we do not perform our duties as labourers would, with the expectation of reward. We are meant to dedicate ourselves regardless. So how are we to understand the Torah seemingly telling us results of actions?

R’ Shlomo Ganzfried explains how the reward is not actually for the net result of the action performed.

The Gemara in Berachos 28a tells how R’ Zeira was taking a break from his studies, and left the study hall, and decided to sit on the steps outside, so that if a rabbi walked by, he could stand up out of respect and deference, gaining merit while being idle from learning.

The Torah compels and obligates anyone who witnesses an event to not withhold testimony. This also means that a witness can not be paid to testify. However, if someone is hired to investigate something, and they find evidence or the like, they can be still be paid, not violating the above obligation.

This is because they are not being paid for the testimony or presentation of evidence. The payment is for the groundwork and effort put in.

The same is true of the Torah’s affirmation of the reward. R Zeira could gain merit by showing his respect for the wrappings of a mitzva.

This is why it is specifically here that the word עֵקֶב – literally “heel” – appears here. The legwork, the walking, the effort are what matters, and that makes all the difference.

The pasuk says regarding Yom Kippur:

שַׁבַּת שַׁבָּתוֹן הוּא לָכֶם וְעִנִּיתֶם אֶת נַפְשֹׁתֵיכֶם בְּתִשְׁעָה לַחֹדֶשׁ בָּעֶרֶב מֵעֶרֶב עַד עֶרֶב תִּשְׁבְּתוּ שַׁבַּתְּכֶם – It is a complete day of rest for you, and you shall afflict yourselves. On the ninth of the month in the evening, from evening to evening, you shall observe your rest day (23:32).

The Gemara in Pesachim 68b wonders what the Torah means by 9th; since the fast is n the 10th. The Gemara answers that it’s a mitzva to eat on the 9th, and the Torah views someone who eats on the 9th as if he fasted on the ninth and the tenth.

What is the Gemara’s problem and solution? We have this style of date in the Torah previously :

בָּרִאשֹׁן בְּאַרְבָּעָה עָשָׂר יוֹם לַחֹדֶשׁ בָּעֶרֶב תֹּאכְלוּ מַצֹּת עַד יוֹם הָאֶחָד וְעֶשְׂרִים לַחֹדֶשׁ בָּעָרֶב – In the first month, on the fourteenth day of the month in the evening, you shall eat matzos, until the twenty first day of the month in the evening. (Exodus 12:18)

The Gemara did not make a similar deduction from why the Torah says that we should eat matzos on the 14th if in fact we eat them from the 15th. Why particularly by Yom Kippur?

R’ Shlomo Gantzfried notes that there is a halacha that a non-Jew may not keep Shabbos – how could the Patriarchs keep the whole Torah if they were still technically non-Jews?

R’ Pinchas Horowitz explains that there are two types of time spans – the Jewish calendar, where the night begins the calendar date; and the secular calendar, where the day precedes the night. The prohibitin for a non-Jew to keep Shabbos, as explained in Sanhedrin 56b, is keeping Shabbos for a full 24 hours (not necessarily on Saturday; the sane would be true of Monday). However, the source is Genesis 8:22 – וְיוֹם וָלַיְלָה לֹא יִשְׁבֹּתוּ – “day and night shall not recede”. We see that their calendar starts from the morning.

Therefore, the Patriarchs kept Shabbos as we keep it – Friday night and Saturday day. However, on Motzaei Shabbos, the subsequent night of the day they kept Shabbos, they did melocho, when it would still be considered Shabbos for a non-Jew, as his Shabbos would only start in the morning. Thus, they never fully kept a Shabbos of a non-Jew.

With this concept, R’ Gantzfried explains how we can understand why the Gemara is specifically bothered with Yom Kippur and not with Pesach. Pesach was mentioned before the Torah was given – the laws of Pesach are said whilst the Jews were still in Egypt; therefore, 14th at night means the night that actually comes after the day. However, when the Torah commands us about Yom Kippur, we are already in the Jewish calendar mode, thus “9th at night” really means a full 24 hours before Yom Kippur.

Therefore, the question from the Gemara was exclusive to Yom Kippur, and not to Pesach.