Is summer a better time for outdoor advertising because we spend more time, or perhaps more willingly, outside our homes? Is winter worse for outdoor advertising because the day is shorter and we prefer to stay warm indoors? Does the season determine what happens in outdoor advertising, or is it less significant because what matters most is what you say and to whom, with timing being derivative of the goal?
So, is outdoor advertising a seasonal medium or not?

If marketers' activities were so simply dependent on the season, there might be some order - Easter in spring, Christmas in winter, travel ads before vacation times, and hearts for Valentine's Day. To some extent, this is true - just look at stores on November 2nd, when grave candles are massively removed and Santa Clauses start appearing in bulk. But on the other hand - when else should they appear if not then? They can't come in April when Poland is busy grilling.

That's how it is and likely won't change - marketing and advertising exist in parallel with consumer needs. Any other situation would be strange, and that's probably what John Wanamaker had in mind when wondering which half of the money spent on advertising was wasted.
In outdoor advertising, it's usually the money spent on poor design that's wasted, sometimes also on low-quality media. However, timing selection has never really failed. (Yes, last April during Easter it snowed while billboards displayed
"Grilling Season", but do you really think that despite this, or rather because of it, someone didn't notice that campaign? Besides, in such cases it's hard to blame poor planning - you don't argue with weather in outdoor advertising, you work with it).
In outdoor advertising, as in any other medium, seasonality matters only as far as it aligns with consumer needs. And since in summer the need is to go on vacation, it's very good to plan summer outdoor campaigns. How do we know this?

Because in April and July 2013, we surveyed people traveling by car across Poland. Our primary goal was to check the visibility of ads on large-format billboards, and we also asked about various travel habits.

Here's some of what we learned:

1. 88% (April) and 86% (July) of adult Poles travel by car across Poland.
2. 86% (April) and 88% (July) of respondents notice advertising billboards placed along roads.
3. 49% (April) and 55% (July) of respondents remembered a specific advertisement/brand.
4. 2/3 of people planning trips for long weekends say that the spring-summer season is an opportunity for more frequent travel.
5. Most vacation trips are planned for July (42%) and August (44%).

The season for well-planned outdoor advertising lasts all year.
Quod erat demonstrandum.