If you always menstruate before day 30 of your cycle, you are correct that you will observe only the veset haflagah. This is not because you have a veset kavua, but because you get your period before reaching your other veset days. If you ever do get your period later, you will keep yom hachodesh and onah beinonit like any woman who does not have a veset kavua.

Everything you wrote shows a correct understanding of the concept of veset kavua. A woman with a veset kavua gets her period either at regular intervals or on regular dates on the Hebrew calendar. This veset needs to be precise not just to the date, but to the daytime or night-time onah.

However, when a woman’s period consistently begins within a two-or three-day window, some halachic authorities (including our founding Rabbinic Supervisor, Rav Yehuda H. Henkin) consider the entire two or three days to be an extended veset haflagah.

Our current Rabbinic Supervisor, Rav Kenneth Auman, follows the view that if there is even slight variation in timing, even between day and night of the same day, the couple do not need to observe an extended veset. However, this ruling calls for added caution around relations. Relations would be permitted on the additional days beyond the single onah of the veset haflagah, so long as there is no sign that the period is imminent. One recommended way to clarify this is by wiping externally with a tissue or toilet paper prior to relations (not immediately after urinating), to see if there is any bleeding.
 
In your case, where you consistently get your period on day 27 or day 28, you should observe the single onah of your veset haflagah according to your last period, and take precautions on the additional onot of days 27 and 28.

Please feel free to follow up if this is unclear!

This response was update on 21 December, 2025.