Polk County sits inland, away from both coasts, which means it loses the sea breeze that makes coastal Florida bearable in August and gains cooler, drier nights in winter than the beach towns get. If you are choosing when to come, the interior calendar is the one to plan against.
Late fall through early spring is the easy answer
From roughly November through April, daytime temperatures are comfortable for walking, humidity drops, afternoon thunderstorms mostly stop, and the outdoor half of this county — gardens, trails, lakes, downtown main streets, festivals — is at its best. This is also when the civic and festival calendar is fullest, because local organizers schedule around the same weather you are.
Summer is for mornings and water
June through September is hot and wet, with storms building most afternoons. That does not rule out a visit; it reshapes the day. Do outdoor things before eleven, plan indoor or shaded stops for the afternoon, and treat the lakes and springs-fed swimming as the point rather than a bonus. Evening events still run, and evening is when the interior actually cools off.
What the ridge changes
The Lake Wales Ridge gives this part of the county elevation, and elevation gives it slightly cooler nights and better drainage than the flatwoods around it. In practice that means a February evening on the ridge can genuinely need a jacket, and a summer morning up on Iron Mountain is noticeably more pleasant than the same hour down at lake level.
Plan against the calendar, not the season
The strongest argument for a specific weekend is usually not weather at all — it is what is scheduled. Festivals, markets, recurring music nights, and one-weekend community events are what turn a pleasant drive into a trip worth taking. Check the events calendar for the dates you are considering and let a scheduled event pick the weekend for you.