I attended an interview on 21 December 2019. I successfully cleared the first round. Due to some time restrictions and unavailability of a panel, HR told me she will schedule a telephonic round but she didn't yet. I want to write a reminder letter to her.
Per your comment on another answer:
She told me to send reminder letter to her because she was middle of so many activities.
It sounds like you are simply asking how to phrase such a request?
I would go with something like:
Hi HR person,
Thank you for giving me the opportunity to interview with your company.
Per our previous communication, you requested that I remind you to schedule a phone interview with me.
The only day which could be problematic for me is Thursday from 9am-1pm so please feel free to suggest any other time.
I hope all is well during this busy holiday season and look forward to the next phase!
Sincerely, Riya sen
If your interview was Saturday then I would wait until Tuesday morning at the earliest to send the reminder.