Late Hefsek Taharah for a Bride

A couple are getting married on Sunday in Israel (they are arriving on Wednesday). The bride has been on hormonal birth control to regulate her cycle for the past 2 months. Her last cycle began on 3 Av during the day and she is absolutely positive that it ended on the evening of 9 Av (as the fast began Monday night) as she was checking to see if her cycles are regulated. Her appointment for immersing in the mikveh is for Thursday night. But she did not do the hefsek tahara until Sunday before evening because she was focused on the wedding date. The formalities and preparations on Sunday of the wedding preclude her going to mikveh immediately before the wedding. The bride would like to know if there is any possibility for her to keep the appointment for this Thursday or if necessary go Saturday night. I was wondering if there is a tzad lhakel from siman 192 and the shitot of Ravyah and Rashba Thank you

Shalom!

Thank you for your question.

In this particular instance, the bride should toivel Sunday morning.