This upcoming Sunday, Rewind TV will have a 13-hour marathon of Welcome Back, Kotter.
The marathon will start on August 11th at 10am ET and will end at 11pm ET.

List of episodes, all times Eastern:

10am-S1E3: "Welcome Back" (OAD: September 23, 1975)
10:30am-S1E8: "One of Our Sweathogs Is Missing" (OAD: October 28, 1975)
11am-S1E5: "The Election" (OAD: October 7, 1975)
11:30am-S1E9: "Mr. Kotter, Teacher" (OAD: November 4, 1975)
12pm-S1E1: "The Great Debate" (OAD: September 9, 1975)
12:30pm-S1E20: "The Telethon" (OAD: February 12, 1976)
1pm-S1E21: "Kotter Makes Good" (OAD: February 19, 1976)
1:30pm-S2E1: "Career Day" (OAD: September 23, 1976)
2pm-S2E2: "Inherit the Halibut" (OAD: September 30, 1976)
2:30pm-S2E17: "Chicken à la Kotter" (OAD: January 27, 1977)
3pm-S2E9: "Hello, Ms. Chips" (OAD: December 2, 1976)
3:30pm-S2E8: "Sadie Hawkins Day" (OAD: November 18, 1976)
4pm-S2E10: "Horshack vs. Carvelli" (OAD: December 9, 1976)
4:30pm-S2E14: "Caruso's Way" (OAD: January 6, 1977)
5pm-S2E21: "Radio Free Freddie" (OAD: February 17, 1977)
5:30pm-S3E6: "Just Testing" (OAD: October 6, 1977)
6pm-S3E12: "Kotter for Vice Principal" (OAD: November 17, 1977)
6:30pm-S3E13: "Swine and Punishment" (OAD: November 24, 1977)
7pm-S3E17: "Meet Your New Teacher: Batteries Not Included" (OAD: January 5, 1978)
7:30pm-S3E19: "Epstein's Term Paper" (OAD: January 19, 1978)
8pm-S4E11: "Frog Day Afternoon" (OAD: November 25, 1978)
8:30pm-S4E8: "X-Rated Education" (OAD: October 28, 1978)
9pm-S4E1&E2: "The Drop-Ins" (OAD: September 11, 1978)
10pm-S4E22: "I'm Okay, But You're Not" (OAD: June 1, 1979)
10:30pm-S4E23: "The Breadwinners" (OAD: June 8, 1979)

All times CDT:

9:00 AM
Welcome Back, Kotter "Welcome Back"
9:30 AM
Welcome Back, Kotter "One of Our Sweathogs Is Missing"
10:00 AM
Welcome Back, Kotter "The Election"
10:30 AM
Welcome Back, Kotter "Mr. Kotter, Teacher"
11:00 AM
Welcome Back, Kotter "The Great Debate"
11:30 AM
Welcome Back, Kotter "The Telethon"
12:00 PM
Welcome Back, Kotter "Kotter Makes Good"
12:30 PM
Welcome Back, Kotter "Career Day"
1:00 PM
Welcome Back, Kotter "Inherit the Halibut"
1:30 PM
Welcome Back, Kotter "Chicken a la Kotter"
2:00 PM
Welcome Back, Kotter "Hello Ms. Chips"
2:30 PM
Welcome Back, Kotter "Sadie Hawkins Day"
3:00 PM
Welcome Back, Kotter "Horshack vs. Carvelli"
3:30 PM
Welcome Back, Kotter "Caruso's Way"
4:00 PM
Welcome Back, Kotter "Radio Free Freddie"
4:30 PM
Welcome Back, Kotter "Just Testing"
5:00 PM
Welcome Back, Kotter "Kotter for Vice Principal"
5:30 PM
Welcome Back, Kotter "Swine and Punishment"
6:00 PM
Welcome Back, Kotter "Meet Your New Teacher: Batteries Not Included"
6:30 PM
Welcome Back, Kotter "Epstein's Term Paper"
7:00 PM
Welcome Back, Kotter "Frog Day Afternoon"
7:30 PM
Welcome Back, Kotter "X-Rated Education"
8:00 PM
Welcome Back, Kotter "The Drop-Ins"
8:30 PM
Welcome Back, Kotter "The Drop-Ins"
9:00 PM
Welcome Back, Kotter "I'm Okay, but You're Not"
9:30 PM
Welcome Back, Kotter "The Bread Winners"

Damm didnt even know about this while my friendly subscription was still active