If it's any help, our band use a Google Calendar shared between us all. If it is in the calendar, the gig is booked. Any dates people can't do are in the diary too, but when a gig is to be booked there is a WhatsApp discussion. It's something we just do and have gotten used to having a chat about..well, everything and wise really.
A second layer of checking is that it's on our website. In case of technical problems, I use IFTTT to add the dates to another non-Google calendar that I view daily.
This may seem over the top, but we are deadly serious about being a great band that people will want to book.