Hello
We use the plural form for each day of the month, except from the first one.
For example:
στις δύο Μαΐου = on the 2nd of May
στις πέντε Μαΐου = on the 5th of May
στις έντεκα Μαΐου = on the 11th of May
BUT στη μία Μαϊου= on the 1st of May
We also use the singular form, when we talk about the sequence of the days.
For example:
τη δεύτερη μέρα του Μαΐου = on the 2nd of May
την πέμπτη μέρα του Μαΐου = on the 5th of May
την ενδέκατη μέρα Μαΐου = on the 11th of May
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.