I have a veset kavua where I get my period on exactly the 33rd day. For two months, the cycle was off: The first month it was day 32 and the second month was day 34. On the third month, I began spotting on day 33 but the actual flow didn't start until that night, which is already day 34. Can I consider my period to have begun on day 33 so I can retain the veset kavua?
In order for a veset kavua for a given interval to be uprooted, the veset day has to pass without a bloodflow or a bloody bedikah for three consecutive cycles. We do not usually base vesatot on bedikah cloths, but a bloody one means that the day is not uprooted.
The first cycle: Assuming you had bleeding on day thirty-three the first month, the first month did not in any way uproot your veset kavua, even though the bleeding began earlier.
The second cycle: If you had no bloodflow and no bloody bedikot on day thirty-three of the second cycle, then you did have one act of uprooting towards the three needed to uproot a veset kavua.
The third (most recent) cycle: If you found blood on a bedikah cloth on day thirty-three this past cycle, it did not uproot your veset kavua in any way and cancelled any uprooting from cycle two. But if you only had staining that didn't make you niddah on day thirty-three this past cycle, it could count as a second instance of uprooting. Even in this case, you would still have your veset kavua (for now).
When a woman has a veset kavua for interval, if the interval changes, she is required to observe the new interval day in addition to the old one. The interval is assessed from the beginning of a flow, not from staining. So this coming cycle, you will observe night thirty-four as a veset she-aino kavua.
(Note: Normally, the yom hachodesh would also be observed as a veset she-aino kavua. In this case, since you have established a consistent pattern of menses at intervals longer than one month, you need not observe yom hachodesh.)
To summarize: You still have a veset kavua for day thirty-three. In addition to your veset kavua, you will also observe night thirty-four as a veset she-aino kavua this coming cycle.
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