120:4 Before one recites the bracha, he should know over what he is saying it, i.e., he needs to know what day of the Omer it is. (He also needs to understand the language in which he is counting. Even Hebrew doesn’t work if one doesn’t understand what he is saying – Mishnah Brurah 489:5.) However, if he did not know but he started to recite the bracha with the intention to count whatever day he would hear from another person, he still fulfilled his obligation. Also if one recited the bracha intending to count the fourth day and, after reciting the bracha, he realized that he needs to count day five, he can count five and he does not need to repeat the bracha. Similarly, if he made a mistake with the actual count, such as if he should have counted the sixth day but instead he counted the fifth day, if he remembers immediately and counts properly, he does not need to repeat the bracha. If he paused even slightly, then he must repeat the bracha.
(Editor’s note: The following halacha refers to the practice in some communities to recite a liturgical poem called a maaravis as part of the evening service.)
120:5 If the first night of yom tov falls on Shabbos, when we do not say the maaravis, then we say the maaravis of the first night on the second night. The exception is Pesach; even if the first night falls on Shabbos, on the second night we say the maaravis of that night. This is because it refers to the harvesting of the Omer grain, which was done on that night.