Every evening towards the end of minyan at this time of year we “count the Omer.” The source of this practice is in this week’s Parasha, Leviticus 23:15: “From the day after the Sabbath, the day you brought the sheaf of the wave offering, count off seven full weeks. Count off fifty days up to the day after the seventh Sabbath, and then present an offering of new grain to the LORD .”
What happens if you miss a day? According to Halachot Gedolot, one of the earliest halachic collections which dates from the 8th century or so, that’s it -- you’ve missed the mitzvah and you are done for the year. Why? Because the text says to count seven full weeks. Having missed a day, you can’t fulfill the mitzvah.
But Hai Gaon, who wrote at about the same time, says no, we are to “count off fifty days” and thus every single day is its own individual mitzvah. So if you missed a day, you missed the mitzvah of that particular day but that is all. Just as failing to put on tefillin one day doesn’t mean you can never put them on again, or missing one Shabbat doesn’t mean you can never observe Shabbat again.
Normative halacha follows Hai Gaon, although technically one is supposed to continue the count without saying the blessing if you missed a day. In our era, when people are sometimes selective or inconsistent in their observance, we need to remember that every mitzvah is independent of every other mitzvah. Whatever mitzvot you observe are worthwhile and stand on their own; failure to observe one does not invalidate the others.