Great question! When there are multiple custom rates (including recurring custom rates) set for particular dates, the custom rate with the shortest rate period will be applied. So, if your one time rate for 2019 has a shorter rate period, it will be used and you won't need to turn off "Repeat Annually" on your usual seasonal pricing. If your seasonal rate that repeats annually has a shorter rate period, then you should turn off "Repeat Annually" to accommodate the special rate in 2019.
For more on rates and which rates take precedence see this help article.