Laws of the Omer

(א) בליל שני אחר תפלת ערבית מתחילין לספור העומר ואם שכח לספור בתחלת הלילה הולך וסופר כל הלילה ומצוה על כל אחד לספור לעצמו וצריך לספור מעומד ולברך תחלה וסופר הימים והשבועות כיצד ביום הראשון אומר היום יום אחד (בעומר) עד שמגיע לשבעה ימים ואז יאמר היום שבעה ימים שהם שבוע אחד (בעומר) וביום שמיני יאמר היום שמונה ימים שהם שבוע אחד ויום אחד (בעומר) וכן עד שיגיע לארבעה עשר יאמר היום ארבעה עשר ימים שהם שני שבועות (בעומר) ועל דרך זה מונה והולך עד מט יום:

Shulchan Arukh, Orach Chayyim 489:1

(1) On the second night after the evening prayer, we begin to count the Omer. And if someone forgot to count, [that one may count] from the beginning of the evening onwards. He may account all night. It is a mitzvah for each person to count for themselves. He needs to count standing and bless before. He should count the days and weeks. How? On the first day he should say “today is day one of the Omer”, until he arrives to seven days. [At which point he should say], “they are one week of the Omer.” And on the eighth day he should say “today is eight days and they are one week and one day of the Omer.” And also, when he arrives to the 14th day, he should say “today is 14 days, they are two weeks of the Omer.” And in this way he should count and onwards until the 49th day.

אורח חיים תפ״ט:ב׳

(ב) אם טעו ביום המעונן ובירכו על ספירת העומר חוזרים לספור כשתחשך והמדקדקים אינם סופרים עד צאת הכוכבים וכן ראוי לעשות:

Shulchan Arukh, Orach Chayyim 489:2

אורח חיים תפ״ט:ב׳-ד׳

(ב) אם טעו ביום המעונן ובירכו על ספירת העומר חוזרים לספור כשתחשך והמדקדקים אינם סופרים עד צאת הכוכבים וכן ראוי לעשות: (ג) המתפלל עם הצבור מבעוד יום מונה עמהם בלא ברכה ואם יזכור בלילה יברך ויספור. הגה: ואפילו ענה אמן על ברכת הקהל אם היה דעתו שלא לצאת יחזור ויברך ויספור בלילה (בית יוסף בשם רשבא): (ד) מי ששואל אותו חבירו בין השמשות כמה ימי הספירה בזה הלילה יאמר לו אתמול היה כך וכך שאם יאמר לו היום כך וכך אינו יכול לחזור ולמנות בברכה אבל קודם בין השמשות כיון שאינו זמן ספירה אין בכך כלום. הגה: וכשהגיע הזמן אסורים לאכול עד שיספור ואפילו התחיל לאכול פוסק וסופר מיהו אם התחיל לאכול קודם שהגיע הזמן אינו צריך להפסיק אלא גומר אכילתו וסופר אחר כך (דברי עצמו למאן דאמר ספירה בזמן הזה דאורייתא):

Shulchan Arukh, Orach Chayyim 489:2-4

(2) If they (the people who are counting) made a mistake on a cloudy day and they blessed on the counting of the Omer, they should go back to count when it got dark. Those who are punctilious, they do not count until the stars come out [namely, three stars], and this is a proper thing to do. (3) The one who prays with the community during the day, he should count with them without a blessing. And if he remembers at night, she should bless and count. GLOSS: And even if he answered “amen” to the blessing of the community, if he had the awareness that he was not fulfilled [of his obligation], he may go back and he may bless and count that night. (4) The one who asks his friend [during the period of time] between sunset and when the stars come out, “how many days of the counting [of the Omer] is it tonight?”, he should say to him, “yesterday was such and such.” Because if he had said to him, “Today is such and such,” he would not be able to go back and count with a blessing. However, [if he answered back] before [the period of time] between sunset and when the stars come out [he can go back and count], since it wasn’t the time of the counting of the Omer [which is when it is night time] and it doesn’t count for anything. GLOSS: And when the time arrives [to count], it is forbidden to eat until he counts and even if he began to eat, he should stop and count. However, if he began to eat before the time [to count] arrived, it is not necessary for him to stop except [when] he finishes his eating and counts afterwards.

אורח חיים תפ״ט:ה׳

(ה) אם אינו יודע החשבון ופתח אדעתא דלסיים כמו שישמע מחבירו ושתק עד ששמע מחבירו וסיים כמוהו יצא:

Shulchan Arukh, Orach Chayyim 489:5

אורח חיים תפ״ט:ה׳-ז׳

(ה) אם אינו יודע החשבון ופתח אדעתא דלסיים כמו שישמע מחבירו ושתק עד ששמע מחבירו וסיים כמוהו יצא: (ו) אם פתח ואמר ברוך אתה ה' אלהינו מלך העולם אדעתא דלימא היום ד' שהוא סבור שהם ד' ונזכר וסיים בה' והם ה' או איפכא שהם ד' ופתח אדעתא דלימא ארבעה וטעה וסיים בה' אינו חוזר ומברך: (ז) שכח ולא בירך כל הלילה יספור ביום בלא ברכה:

Shulchan Arukh, Orach Chayyim 489:5-7

(5) If he does not know [which day of the Omer it is], and he opens [his mouth to bless on the day] with the intention to finish [the blessing] like [what] he will hear his friend [say as he ends the blessing] , and he stops talking until he hears his friend, and then finishes [the blessing] like him, then he has fulfilled his obligation [to count the Omer]. (6) [The Shulchan Aruch here will present two cases in which the counter thinks one number and says another, and in both cases, the counter cannot go back and bless the day correctly]: [For example]: If he opens [his mouth to bless on the day] and says, “Praised are you, God, Our God, king of the universe” with the intention to say, “today is four [days of the omer]“, because he thinks that the count is four, and then he remembers [the correct day] and finished [the blessing] with [the number] “five” and the [days are indeed five] Or on the contrary [another case], that [the count of the days] is four and he opens with the intention to say four and he was mistaken and finished [the blessing] with five [days of the omer], he should not go back and bless [on the day correctly]. (7) If he forgot [to count] and did not do the blessing all night, he should count during the day without a blessing.

אורח חיים תפ״ט:ח׳

(ח) אם שכח לברך באחד מהימים בין יום ראשון בין משאר ימים סופר בשאר ימים בלא ברכה אבל אם הוא מסופק אם דילג יום אחד ולא ספר יספור בשאר ימים בברכה:

Shulchan Arukh, Orach Chayyim 489:8

(8) If he forgot to bless on one of the days, between the first day [of the omer] and the rest of the days, he should count the rest of the days without a blessing. But, if he uncertain if he skipped a single day and didn’t count, he should count the days with a blessing.

Shulchan Arukh, Orach Chayyim 493:1-3
[The time period between Pesach and Shavuot is called the Omer]. The minhag is not to marry a woman between Pesach and Shavuot until Lag baomer (33rd day) because during that time the students of Rabbi Akiva died. But to get engaged it’s allowed. And if someone does marry we don’t punish him and after Lag Baomer it is all permitted.
2. The minhag is not to get a haircut until Lag baomer because then the students stopped dying. But one should not cut one's hair until the 34th in the day unless the 33rd is erev Shabbat, then you may cut hair on the 33rd because of honoring the Shabbat. GLOSS: In these states we don't follow his position rather we cut hair from the day of the 33rd and add some extra joy and we don't recite the prayer of Tachanun. But you don't cut hair until the day of the 33rd not the night before. However if the 33rd falls out on Sunday we have a custom to cut hair on the Friday before out of respect for Shabbat. Someone celebrating a Brit Mila may cut his hair duing the Sefira in honor of the festivity.
3. Some have the custom to cut their hair on Rosh Chodesh Iyar but they are mistaken. GLOSS: however in many places some have the custom to cut their hair until Rosh Chodesh and they should not cut their hair form the 33rd day and onwards , even though it is permitted to cut on the 33rd day itself. Those who do cut their hair after Lag Baomer do not make any exceptions after Pesach until the 33rd day. One city should not have customs of both sides in the same city because of the prohibition of 'do not make groups'. And of course one should not be lenient with both at the same time.