-During the first week of sefirah, if it’s after Sunset, and before you counted Sefirah, and someone asks you “What night of Sefirah is it tonight?”, you should respond by saying “yesterday’s count was such and such” (i.e. on the 5th night of Sefirah say “yesterday was the 4th day of the Omer” ).
If you mistakenly answered saying “Tonight’s count is such and such” , you may no longer recite that night’s count with a Bracha, as your response to that person is considered an adequate “counting”. (See Mishna Berura 489:22, and Sha’ar HaTzion ibid. 23 and 26)
The above applies even if you just said “Tonight is 6”, and didn’t say “Tonight is the 6th night of the Omer” (Mishna Berura 489:210)
However, if someone asks you the count, and you simply respond by saying “6”, and don’t preface it by saying “Today is”, it isn’t considered a count and you may go ahead and count again with a Bracha. (Mishna Berura 489:20)
-Once the second week of counting starts, when we count the days as well as the weeks, if you responded to the inquiry saying “Tonight is 9”, as long as you didn’t add the information “…Which is one week and 2 days” you haven’t satisfied your obligation, and may go ahead and count with a Bracha. (Mishna Berura Ibid:22 and Sha’ar HaTzion ibid. :28)
Therefore, if you tell someone on the night of the 33rd day of the Omer that “Tonight is Lag B’Omer”, there is no need to worry, and you are still required to count the Omer with a Bracha. (This is a very common occurrence, and many mistakenly do not make a Bracha after it happens.)