Performing Melacha prior to counting the Omer at night:

1. Before sunset:[1]

Men, being that they are obligated in the mitzvah of Sefiras Haomer, are forbidden to begin performing Melacha starting from one half‑hour prior to sunset, just as was explained above regarding the prohibition of eating, and as applies prior to all time dependent Mitzvos. However, just as we rule with regard to eating, there are certain leniencies in this matter, such as if one began the Melacha prior to the half‑hour, or if one appointed a shomer (reminder) to ensure that he will not forget to fulfill the mitzvah, it is permitted, as explained above in Halacha 4. Nevertheless, from sunset itself and onward, one is not to engage in Melacha until after fulfilling the mitzvah even if one of the above allowances apply, as will be explained next. [All of the above applies exclusively to men, who are obligated in the mitzvah, and are therefore subject to the restrictions against eating and performing Melacha during the times discussed above. Women, however, since they are not obligated in the mitzvah, are not included in these restrictions and may perform Melacha until sunset itself, as will be explained in the following section. However, this only applies to those women who have not accepted upon themselves the mitzvah of counting Sefiras HaOmer as an obligation. However, those women who have accepted this mitzvah upon themselves as an obligation are required to follow the same laws as men, with regard to abstaining Melacha.[2]]

1. From sunset:[3]

It is accustomed for both men[4] and women to avoid doing work/Melacha during the entire period of Sefiras HaOmer, from the time of sunset until after counting the Omer.[5] (Possibly, those women who are not accustomed to count the Omer must refrain from work throughout the entire night.) [For this reason, all women should count the Omer and should continue to count the Omer nightly even if they can no longer say a blessing due to missing a day, as otherwise they cannot do Melacha until morning.[6] This applies even if they appoint a Shomer, and even if they started the Melacha before nightfall.] [Despite the above ruling, many are not accustomed to abstain from Melacha during this period of time and there are Poskim[7] who have defended their actions.]

Performing Melacha Prior to Counting the Omer at Night

1. Before Sunset

1. From Sunset and Onward

Q&A – What Type of Work Is Restricted?

[1] M”B 489:24; Kaf Hachaim 489:67; Nitei Gavriel Pesach 3 26:2  

Tzaruch Iyun: This matter requires further analysis, as to why this prohibition is omitted from the Shulchan Aruch of the Mechaber, as well as from the Magen Avraham, Taz, and Shulchan Aruch Harav, all of whom make no mention of a prohibition of engaging in Melacha together with the prohibition of eating. Even according to the Alter Rebbe, who — in contrast to the Mechaber who only mentioned women— does mention this prohibition in chapter 493, this is stated explicitly only with regard to after sunset and not from one‑half hour beforehand, and only as a custom applicable to both men and women. Nevertheless, whatever the rationale behind these omissions may be, it is evident that the Acharonim maintain that the same restriction indeed applies beginning from one half‑hour prior to sunset, similar to the prohibition of eating. The practical difference between these cases would relate to the various leniencies discussed earlier — such as where one appointed a shomer to remind him, or where one began the activity prior to the prohibited time.

[2] See regarding Shofar: Ashel Avraham Butchach 689 and Mahadurah Tinyana 592

[3] Admur 493:9; Michaber 493:4

[4] In the Michaber 493:4 men are omitted, and the custom is only brought regarding women. Nevertheless, the Kneses Hagedola adds that the custom applies likewise to men, and so rules Admur ibid. Some explain the reason the Michaber omitted the custom for men is because men are in any event forbidden from doing Melacha from sunset until they count, as is the general law regarding any Mitzvah which arrives, such as Bedikas Chameitz, and prayer. [Olas Reiyah 52]

[5] The reason: This is hinted to in the verse “Sheva Shabasos” and Shabasos has the same terminology as the word rest, hence implying that from the time of the counting of Sefira, which is from sunset, until one counts, one is to rest from doing Melacha. [Admur ibid; this is the 2^nd reason mentioned in the Tur and is based on the ruling of Taz; Mamar Mordechai 493:8; Nehar Shalom 493:3; Kitzur SHU”A 120:10]

Other reasons and opinions mentioned: The students of Rebbe Akiva were buried during the period of Sefira, after sunset, and it is accustomed to avoid doing Melacha while a Levaya/funeral is in procession. Based on this reason, one is to abstain from Melacha the entire night, even after counting. [1^st reason in Tur and M”B 493:19; See Mishneh Halachos 8:215] Another reason: The period between Pesach and Shavuos is similar to Chol Hamoed, as Shavuos is similar to Shemini Atzeres of Pesach, and thus in commemoration of this the Sages enacted to avoid Melacha after sunset. [Yaavetz in Mor Uketzia based on Ramban Vayikra 23:36]

[6] Olas Reiyah 52

[7] Chok Yaakov 493:12 writes he has not seen people careful in the above custom; Beis David 271, brought in Kaf Hachaim 493:43, rules there is no obligation to follow this custom, and those which were not accustomed to do so are not to be protested. See Aruch Hashulchan 493:9 that even today some women follow this custom; Mishneh Halachos 8:215; Piskeiy Teshuvos 493:16

[8] Siddur Yaavetz; Kaf Hachaim 493:45

[9] Kovetz Mibeis Levi 1:57; Piskeiy Teshuvos 493:16

[10] Ashel Avraham Butchach, brought in Piskeiy Teshuvos ibid