ames: 6pm is an approximation. The sun was created at sunset on Tuesday evening (which is the exact beginning of Yom Revi’i, since there is no safek of bein hash’moshos to God). As I explained in my earlier posts, we use approximation in calculating this event because an exact calculation is overly complex.
The approximation is the Tanna, Shmuel’s. He said that a year is exactly 365 1/4 days. Therefore, the extra quarter day means that the year begins and ends first at 12 am, then 6 am, then 12 pm, then 6 pm.
Also, the sun was created (according to Shmuel’s shittah) at the vernal equinox. Roughly speaking, that means that there was exactly 12 hours of day and 12 hours of night at creation. So 6 pm is a reasonable time to call sunset, and 6 am was the first sunrise (and there was no DST).
yossiea: Jews didn’t gather for mass rock concerts 200 years ago either. If Jews can get together for that, they can get together for mitzvos too.