"Hello, Elani! Poseideon 2 has just begun, and I'm wondering whether we should all just repeat the festivals from last month (Poseideon )? I hope you can answer my question! :)"
I'm going to try to give you an answer you can work with, but there is a lot unclear, as you will see. A bit about the calendar first: I have said before that a full lunar year is 354 days long. Because the earth rotates around the sun in roughly 365 days, an extra month was inserted into the twelve-month calendar every few years--usually every third year. This month was usually a repeat of the previous month, most frequently Poseideon, but there are references to repeats of Hekatombaion, Metageitnion, Gamelion, and Anthesterion. How long this month was, depended on the previous years.
Us users of the Gregorian calendar tend to see dates (and time) as constants--they can't be changed. February second will always be February second (even if we have our own shift by adding a day to it every four years). The ancient Hellenes saw the calendar as being extremely flexible. They had a tendency to repeat days to suit their needs, usually to postpone the arrival of a certain date. Assembly meetings, for example, were not held on festival days, so if the meeting was urgent, the previous day was repeated and the festival day postponed. A standard extra month would have been thirty-three days long, but it rarely was; days would have been inserted throughout the previous years, eating into the extra month.
It is unknown if the festivals which fell in this month were repeated as well, if other festivals were held, or if no festivals were celebrated at all. There simply isn't a remaining source to tell us. As such, I cant answer your question with ancient sources... but I can give you my personal thought process on the subject.
In general when a day was added to the calendar in order to push back a day, the day was not 'counted'. It bore the name of the previous day and was treated as a regular day--a non-festival day where meetings could take place and state affairs handled. Now, of course, an intercalculated month is different than an added day, but fact remains that this was a period of time that was named after the previous period of time. It thus stands to reason that this month was void of festival just like the added days. As such, I tend not to repeat festivals. I do observe the standard, recurring events like the Noumenia of the Mên kata Theion.
There is an exception, though. Research into the Poseidea suggests that instead of being held on the eighth of the month, the festival was actually held on the equinox. If this was the case, the festival would sometimes be celebrated in the repeated month (like this year, pushed into it because it landed on the Hene kai Nea, which would is auspicious).
I hope this answers your question as much as possible. Unfortunately, some things about our religion are truly just gut instinct, and educated guesswork. Though much has survived, many sources were lost to us. those that are rebuilding do so slowly but steadily, and sometimes we have to take liberties--like with this.