Letters weren't only for cross-continental communication.

Before electronic communication you'd send a letter or just show up unannounced.

If this is about that period of human history where we had long-distance transportation (ie railroads) but didn't yet have mass communication infrastructure that isn't the postal service -- so 1830s to 1860s -- then I think the answer is to just plan to meet the other person at a certain place every month.

To use modern parlance, put a recurring meeting on their calendar.

Sending letters via post to friends in the same city wasn’t uncommon—but beyond that, you could leave messages at common locations. Like if you both go to the same shop once a week, you could leave messages for each other with the shopkeeper.

If you're going that far back means those two friends in the same city were probably going to church (or some other religious gathering) fairly regularly and would likely see each other fairly often. Enough to be able to make other plans during the week. And yes, can't tell someone you're running late or not showing up.. if you're late you're late, if you don't appear then that'll be a conversation next time at church.

Beyond that since they're in the same city those friends likely go to the same pub(s) or other places and see each other that way.

Before landlines?

As in prior to 1876?

hop on a donkey, slap its ass, get lost in a forest, get eaten by a bear.

eeeazeeeee....

Letters are only for those cross-continental, cross-oceanic relationships.

Letters were commonly written for people in the same city. Some posh neighborhoods in major cities would have hourly mail service.

Depends on where you lived.

In some places, you’d leave a note with someone you knew the recipient was going to see that day. In some places, note passing was quite an artform, including special paper folding to protect the contents from prying eyes.

In Paris, they had the tubes — the entire city was plumbed with vacuum tubes, and you could write a note or even pack a small object into a capsule with an address, drop it in your local receptacle, and it would zip across town in minutes to the recipient address.

In other places, markings on trees, mirrors, flags, smoke signals and various musical instruments and bells have been used.

In school. We used to plan where we want to go on certain date and time and just meet there. Admittedly this was still a time with a landline and payphones and my part of the world was just getting cellphones but that's for middle to upper class.

Go out earlier is a practice that I think it is not done anymore. If I have a meeting at 4pm and I know that it takes me 30 minutes to get there, I go out of my house one hour before. This is for answer your doubt about how would you let know to your friend that you will be late, you weren't able, the best you can do is be preventative.

Imagine you send a letter to a fellow who lives overseas, you expect to see him september the 3rd in a concrete place. I can bet that person would arrive like a month before and wait the right time. (Obviuosly would not wait for you in that very place, that person can stay in a hotel or whatever).

You'd show up at their house earlier that day to say, I'm going to be at X place at Y time, I hope you can join me?

I feel like those were the days where you could have true friendships in society, not “having friends to send memes”.

Um, excuse me? You need to get out of your internet bubble. Tons of people still have friends in real life - most of them, actually. If you don't, and you wish you did, then you have an unusual problem and you should start working on solving it asap

My parents lived in a part of the world where they didn't always have phones. Dad lived on a farm, mom lived in town. School together, Church together. After school, Dad would go "help around the house" at mom's. Or vice versa. Once they got old enough, one or the other would go to the city in the family car for shopping, and bring the other with. Go see a movie, grab a bite to eat together.

Just arranging it the last time in person. Mail worked just fine to confirm or cancel a bit before since the same city. If you needed things more quickly, couriers was one way. There were also a lot third spaces and people met out and about more and more often. They might see each other every Sunday at the same church for instance.

Up until I was in uni, even payphones didn't matter most of the time since there's no guarantee both parties are going to be near one and no normal person had a pager. If you were going to a business that had a phone, you could look up their number and call them to put one of your buddies on the line or at least send a message (see the running gag on the Simpsons where Bart calls the bar to ask for someone).

We also just waited a bit and if they didn't show, we went on with our plans.

Before landlines? You know those have been around for over 100 years?