Here is how we kept details of each wedding client straight back in the days before personal computers:
Each job had a large envelope (about 9 x 12). We pre-printed 5 x 7 cards with places to list all the pertinent specifics - dates, times, locations, names and phone numbers. Inside the envelope was kept a copy of the contract, receipts, the photographer's check list and any other pertinent information. The checklist included notes about any special relatives expected at the wedding they wanted photographed, whether there was any divorce/remarriage among the parents and how that would be handled, and any other special notes about the day. Anytime there was a phone call or meeting with the clients, any updates or additional info would be noted and retained in the envelope. We stored all the envelopes for future weddings in a file drawer by wedding date.
Since weddings may be booked as much as a year or more in advance, and a busy wedding business might have a lot of future bookings, it's impossible to remember all this stuff. Your booking calendar marks off the dates and times for which you are committed to a job. Everything you need to know is in the envelope to be reviewed before starting out on the job.