Who's watching what depends on where you are and which networks are airing what season.
Confused yet?
I'm in Maryland. In May 2002 UPN and FOX networks finished their run of S4, which was then rerun all summer and into the fall. A few weeks later cable station SciFi began their run of the first 10 eps of S6. S5 had not been shown here, except for those who got the premium station Showtime.
UPN and FOX are now running S5 -- same episode on each channel on Saturdays. SciFi is running four S1 eps in a row on Monday nights and were running S6 eps on Friday nights, but have been in reruns of S6 for several weeks.
The SciFi Friday night broadcast has ended with the rerun of "Allegiance" and probably won't pick up again with reruns "Cure" and "Prometheus" until just before they start new eps of S6 in January.
So at least I'm catching up on S5, although I had watched a few key eps on squinty vision download -- had to in order to figure out what was going on in S6 into which we leapt without preparation for the absence of Daniel, the appearance of various S5 characters, and a totally new villain.
With S1, S5 & S6 all running at the same time on various stations, watching SG1 is something of a time travel experience.