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As @moudhy noted (in an amazing thread) it's the autumnal equinox. In this thread I want to show how ancient Mesopotamian scholars determined this and prove that you too can read ancient cuneiform astronomical texts...
The first things to note is that cuneiform numbers are *easy*. It's a sexegesimal (base-60) system with "one" and "ten" markers. There's no place value markers so you just have to infer that from context (but it's usually apparent). 
So where does the data for the equinox and solstices come from? Well funnily enough as part of the standard astrological treatise Enūma Anu Enlil there's a section on length of days and night within a month and throughout the year.
(As a side note this shows how closely interlinked these two practices, astronomy and astrology, were in ancient Mesopotamia. A practice I like to call astronomology.)
Here's an example of length of night calculations from EAE tablet 14. Set aside the signs you can't read, and look for patterns of numbers that ascend and descend. I'm going to highlight these sections in the following tweets... (the tablet is K. 90 in the British Museum)
Hopefully you were able to see these numbers constantly going up. Starting at 1 and ending at 21 at the bottom of the tablet. This is the day of the month (it continues on the reverse). The sign directly before it is "day" and the sign after is the ordinal number marker "th".
These next numbers are trickier, but only because they deal in metrological units. They actually measure weight of water in a water clock. Don't worry too much about the units but notice that they ascend until the 15th day and then descend.
We can tell from the length of the night, 4 (you got that right?) that this table was meant for an ideal winter month with the solstice occuring on the 15th. But many of these ratios were not really accurate and more just handy mathematical shorthand.
So finally let's get to the tablet that shows the equinox. It's a little harder to pick apart but we're going to zoom in on the important parts. This text gives the lengths of day and night for each month of the year. (The tablet is IM 121332 in the Iraq Museum).
In red I've highlighted an alternating sequence of two numbers. Perhaps you can recognize them as 15 and 30 (3 tens). Rather than write out every day of the month the scribe here just highlighted the new moon and full moon for each month.
The signs either side of the numbers are "day" and the ordinal marker from before. The signs to their left are the month names in Sumerian. The next part is trickier...
Each line here follows a very strict paradigm, but it's a bit squished so it's hard to see. First there's a number followed by the signs "day watch" and then another number and "night watch". Those two numbers always add up to 6 (total length of the day).
You should notice the first number slowly going up as the second number descends and then at a certain point they're equal and they reverse behavior (that's the summer solstice. See if you can find "4" and "2" in the first line highlighted.
Here's the part we're interested in, the autumn equinox. This is when the day and night are equal... in this case they're both "3". There's also a bit of text beneath that reads: "day is giving way to night: the days grow shorter, the nights longer" which is quite poetic!
I hope you were able to at least pick out some of the numbers in these texts and see how the patterns can be noticed even with little to no cuneiform training. The early pioneers of the study of Mesopotamian Astronomy were doing just that, reading tablets looking for patterns.
But it also points out how universal certain pursuits are, Mesopotamian scribes were recording data and puzzling with mathematical patterns thousands of years ago, trying to understand the world around them. The commonality is evident in our interest in the equinox even today!
Finally just to cite my sources (and where to go for further reading):
Hunger and Pingree "Astral Sciences in Mesopotamia" Brill 1999, is the de facto guide book to Mesopotamian Astronomy/Astrology
Hunger and Pingree "Astral Sciences in Mesopotamia" Brill 1999, is the de facto guide book to Mesopotamian Astronomy/Astrology
Al-Rawi and George "Enūma Anu Enlil XIV and Other Early Astronomical Tables" Archiv für Orientforschung 38/39 1991/1992 is where the beautiful tablet images came from.
