I school year-round, so I am always planning since my kids finish their subjects at different times throughout the year. We just immediately move into the next book. Many subjects, like grammar & math, don't require me to plan--I order the next book.
I happen to be doing a lot of planning right now, because coincidentally we are finishing up history, grammar, math, Latin, & science by the end of February. We will be starting our new year in those subjects at the end of March. Plus, we're starting TOG for the first time and that is taking a lot of planning.
Luckily, planning is one of my favorite parts of homeschooling. ;)