Point of interest: God never told Israel where to celebrate them. Jerusalem wasn't even in the picture for the original commands.
We had about 60 people camping out the entire 8 days. Some had to go to work throughout the week but came back each night, some came throughout the day when they could, and some camped out only a day or 2.
Actually you are right...not specifically Jerusalem but in the place God told them to:
Deuteronomy 16:16 “Three times a year all your males shall appear before the LORD your God at the place that he will choose: at the Feast of Unleavened Bread, at the Feast of Weeks, and at the Feast of Booths. They shall not appear before the LORD empty-handed.
Assuming that keeping the festivals is part of the current expectations of God, how does he communicate where to keep them?
In Armstrongism, the church decided where they were to be kept. However, how do they derive their authority to speak for God? Or, how does any other organization derive that authority?
Just to make sure it's clear, the Old Covenant isn't in effect anyways but I am wondering how modern day observers reconcile this if they claim it is.
I don't have any issues with individuals who observe these days of their own preference, so it's an irrelevant question for them. This question would only be relevant if someone claims it is a requirement or condition or necessary fruit of salvation. I can see some value in the observances otherwise for commemorative purposes.
I also strongly doubt that Israelites during the Assyrian and Babylonian captivities observed such things. I'd like to see biblical proof if they did. I believe the observances are liked to the Covenant, which is linked to a specific people (Israelites) during a specific time frame (from Moses to Jesus Christ per Galatians 3) in a specific location (Palestine).
Israel was also a theonomy which could be controlled. How is this relevant? If a slave with a Gentile master in the New Testament church was saved, and his Gentile master was not, couldn't you see where that would create an issue if the slave was not allowed by his master to observe such days?
My guess is that Sabbathkeepers who claim it is a requirement would say the slave should simply disobey and forfeit his life. If this is true, why don't we read about this happening in the epistles, or instruction being given in any way regarding this? Also, why is Sabbath-breaking never mentioned in the sin lists of the Gentiles? These are things I had to ask myself as a former Sabbath or festival observer who thought it was a requirement or condition or necessary proof of salvation (which most here probably do not).
I'd like to see someone in the Arctic circle observing the Feast of Tabernacles living in a Sukkot for eight days as the Old Covenant required. I'd also like to see Sabbathkeepers observing the Sabbath in lands where days and nights can last months due to their location near the poles.
Know what Ellen G. White did about such things? She legislated that people couldn't live in those areas