As a course lecturer, should I excuse late assignment submissions if I wrote the assignment deadline slightly wrong?

If the assignment deadline was not shown correctly to the students, then it is your error and they cannot be penalized for being late.

Claiming it is correct in one area while incorrect in another does not absolve you, you caused the confusion so you have to accept late submissions, as long as they arrived on Saturday...

Any submissions on Sunday will, of course, be late.

Re-reading this, it sounds a bit blunt... Probably because I have done exactly the same and had to sort it out after... Peace reigns if you stick to being fair, so giving them the extra time does not usually make much of a difference, except for the recognition from the students.

First thing: The student handed in on the shown deadline. That says everything. Not accepting is not an option in any reasonable way. But to be more specific:

In dubio pro reo (in a case where nothing happened anyway)

Mistakes happen. Like you writing the wrong day for a certain date. Happened once, the "damage" is that students may have a reason to hand in one day later. That's it. And that's basically nothing.

Students are humans too. Most probably he/she really did not pay too much attention, did not look it up properly. Like no one of us would have or normally does in his daily business. And that's fine. Because it was not about life and death.

Let it be like that and accept the hand-in saying that you really wrote the wrong day (and may even apologize for the small mistake, as it may caused some confusion and a little anxiety to the student when he realized it). Even thinking about that incident is too much energy wasted for nothing happened. And be happy that you did not write the date of a test wrong or similar. Keep the mouse a mouse, don't make it an elephant.

Also, not accepting it will result in punishing a (most probably) innocent student. That's just unfair and will leave a very bad impression with the student. No reason to risk that.

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If you occasionally published two different dates as a deadline, you should accept the work until the latest date without penalty. This is what essentially happened: you announced the deadline as 23rd Nov 2018 but also as Saturday, which is not the same day. Go with the latest of two then — this is the best way to be fair in this situation.