I am not aware of their being a specific day that is sacred to Hestia. I’ve found this to be a common theme with Hestia. She is lesser known and mentioned in many myths, but she was such an integral part of worship that she became a part of people’s everyday lives and worship (she’s the keeper of the sacrificial flame and the first part of each offering is given to her).
It sounds like having a specific day would be helpful for you (although it’s not necessary!), so I’d recommend picking the day that feels right and making it your personal sacred day to her!