How to determine the 6th day of the cycle, if sometimes I do not have a period?

Unfortunately, the only way to determine the first day of the menstrual cycle is to wait until the first day of bleeding. The absence of menstrual bleeding causes the menstrual cycle to be extended until it occurs - the cycle is then treated, for example, as 60-day, 90-day, etc. Regulation of menstrual bleeding after discontinuation of hormonal drugs can take several months, and concomitant diseases may further disrupt this process. I enclose an additional article that may be of interest: https://www.gdziepolek.pl/kategorie/leki-na-zaburzenia-miesiaczkowania If you would like to seek additional medical advice, where you can make an online gynaecological appointment at this link: https://www.gdziepolek.pl/telekonsultacja/z-ginekologiem