Hiya Robin!
Welcome to planDisney! I see you are planning your upcoming trip and hoping to make the right decisions with all the information that is out there. I am glad you came here to
planDisney as we are guests like you who love to talk about Disney.
It doesn't matter if you purchase a
single-day theme park ticket or if you add on the
Park Hopper Option because if you have a date-based ticket you do not need to make
theme park reservations. At the time of purchase, you select the date you would like to potentially start using the ticket and will be given a date range where you can use up those tickets. If the tickets that you have are not date-based such as Disney Military Salute Tickets, Annual Passes, Student Group Tickets, or Gifted/Comped Tickets then you will have to make a Theme Park Reservation for the days you wish to visit.
I highly recommend purchasing your tickets well before you arrive to give yourself time to make plans and decisions regarding
Lightning Lane Multi Pass and Single Pass. I checked and found that if I wished to purchase tickets to visit Magic Kingdom Park today I could buy a ticket with no theme park reservation needed. However, I have now lost quite of bit of in-park fun as I waited till the last minute and I definitely will not be buying Lightning Lane Multi Pass as times are now limited to the rest of the day.
I want to note that there is always a chance that on busy days the Parks can reach capacity and even guests with date-based tickets may not be allowed in. This happens especially around the holidays as many like to celebrate these occasions with Mickey and his friends. It is rare but it has happened. The Park that gets filled up the fastest tends to be Magic Kingdom Park followed by EPCOT. I like to look at the
Theme Park Reservation calendar to help make a judgment call about which Park will be busy. I also try to visit the Parks early in the day for those days looking to be busy and just settle in for the day so I don't miss out.
I hope this helps and you have a lovely time!
Happy Planning!
-Nimmy