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Melacha prior to counting:
It is accustomed for both men and women to avoid doing work/Melacha during the entire period of Sefiras HaOmer, from the time of sunset until after counting the Omer. (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.] [Despite the above ruling, many are not accustomed to abstain from Melacha during this period of time and there are Poskim who have defended their actions.]
What form of work is to be avoided?
All activities which require effort and take time to accomplish, such as sewing and weaving, are to be avoided. However, simple house chores such as cooking, sweeping, and cleaning are allowed. Likewise, all Melachos that are permitted on Chol Haomed even according to only one opinion may be performed.
 Admur 493/9; Michaber 493/4
 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]
 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 2nd 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. [1st 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]
 Olas Reiyah 52
 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
 Siddur Yaavetz; Kaf Hachaim 493/45
 Kovetz Mibeis Levi 1/57; Piskeiy Teshuvos 493/16
 Ashel Avraham Butchach, brought in Piskeiy Teshuvos ibid