What is the best time to plant corn and potatoes?

It depends a lot on where you live, because the calendar changes depending on the region. The previous answer is correct about the general months, but there's a detail I think is important: corn is much more tolerant of initial cold than potatoes, so you can sow it in April without much risk, while potatoes really need the soil to be warmer and the late frosts to have completely passed. If you sow potatoes too early, you risk the seeds rotting in cold, damp soil.

What I'd do is check what the average soil temperature is in your area - corn germinates well from 10-12°C, but potatoes want at least 13-15°C. If you have a phone with a simple weather app, it's easy to monitor this. Besides that, the state of the soil is just as important as the timing: before sowing either of the two, work the soil well, remove stones and weeds, and make sure it drains rainwater properly. Potatoes in waterlogged soil is a guaranteed disaster.

For corn specifically, if your region has very hot and dry summers, it might be worth sowing a bit earlier (April) so it catches the spring moisture. As for potatoes, late February or early March is generally safe, but all of this gets better if you observe the local climate of the year in question - every year is different.

You guys are right about the months, but there's an annoying detail I learned the hard way: timing isn't just about the months, it's about the actual soil temperature. Corn germinates properly when the soil is above 15°C, and potatoes prefer between 10 and 20°C. If you plant before the soil gets warm enough (which usually happens in March/April depending on the region), the seed takes forever to sprout and becomes vulnerable to rot.

The hack nobody mentioned: before planting, bury a thermometer in the soil where you're going to plant, leave it there for a week and check what the actual temperature is. Sounds tedious, it is, but it saves you the frustration of seeds sprouting poorly. With corn you've also got the advantage because it's more flexible, but potatoes are a pain in that regard - if the soil is too cold or too hot, your harvest becomes a disaster.

Another thing: if you live in a region that doesn't have hard frosts, you can plant potatoes earlier in January/February without any problem, and corn can go from April onwards. But the thermometer doesn't lie - the months you guys mentioned are just a reference. Soil moisture matters a lot too like you said, but without the right heat nothing sprouts properly.

If you have access to a soil thermometer, it's really worth using because most people don't think about it and end up sowing too early!

Now, one thing I didn't see mentioned: corn can stay in the field way longer than potatoes, so if you have the space, it's pretty cool to sow the corn a bit later (even until July) to make better use of summer rains and avoid pests that show up earlier. Potatoes are more sensitive to timing, that's true, but you also have to account for the fact that harvest quality later depends hugely on not letting the soil get too waterlogged in the first few days - I've seen entire crops ruined by rot because it rained right after sowing.

Don't sow potatoes if the soil is still too waterlogged, because they rot easily - it's a common mistake in wetter years. What nobody really talks about is that besides soil temperature, you need to consider the crop cycle: potatoes usually take about 90 days until harvest, so if you sow in February-March, you harvest around June when it's already hot, and if the summer turns out dry the yield suffers. Corn also needs all that time, but it's less demanding on moisture during growth. It's really worth checking your area's rainfall history and adjusting based on that.