May 21, 1977 has quite the Top 10. Eight of the songs in the Top 10 are songs that hit #1. Might be a record.
It turns out there's a four-way tie. (The Ultimate Music Database, with peak position listings on every weekly chart, made this easy.)
As we know, it first got crazy in late summer and early fall of 1974.
9/7/74...Seven (current #1, one former #1, five future #1's)
9/21/74...Eight (current, 2 former, 5 future)
Then there was the infamous stretch in which the #1 song changed every week for almost three months.
2/1/75...Seven (current, 2 former, 4 future)
3/8/75...Seven (current, 2 former, 4 future)
Moving into 1976, the charts in general began to slow down. The AM top-40 format was losing ground to FM soft-rock, which featured less rotation and slower turnover of playlists. The songs began to linger in the top ten longer.
12/20/75...Seven (current, 1 former, 5 future)
12/27/75...Eight (current, 2 former, 5 future)
1/3/76...Eight (current, 3 former, 4 future)
The next wave came in the wake of "Tonight's the Night" falling from the top.
1/22/77...Seven (current, 2 former, 4 future)
1/29/77...Seven (current, 2 former, 4 future)
Then came a streak of #1 turnover to rival 1974-75.
4/2/77...Seven (current, 1 former, 5 future)
4/9/77...Seven (current, 2 former, 4 future)
4/16/77...Seven (current, 3 former, 3 future)
5/21/77...Eight (current -- "Sir Duke", 3 former -- #2 "When I Need You", #8 "Hotel California", #9 "Southern Nights", 4 future -- #4 "Got to Give It Up", #5 "I'm Your Boogie Man", #6 "Dreams", #7 "Gonna Fly Now")
7/2/77...Seven (current, 2 former, 4 future)
"I Just Want to Be Your Everything" and "Best of My Love" put a temporary end to this revolving-door trend. In fact, by 7/16/77, we were down to four even though "Everything" was still climbing. (There's a post I wrote on Easter about the chart travels of Andy Gibb and the Emotions. It's in the Chart Manipulations thread, on page 4.)
In the fall of 1979, "My Sharona" dropped out of #1, and another free-for-all eventually ensued.
10/20/79...Seven (current, 3 former, 3 future)
11/3/79...Seven (current, 2 former, 4 future)
11/10/79...Seven (current, 2 former, 4 future)
11/17/79...Seven (current, 3 former, 3 future)
12/1/79...Seven (current, 3 former, 3 future)
12/8/79...Seven (current, 3 former, 3 future)
12/15/79...Seven (current, 3 former, 3 future)
Speaking strictly of Casey's first turn as host of AT40, were there any weeks when the top ten contained NO former or future #1's? Glad I asked...
5/7/71..."Just My Imagination" was #15, having just dropped out of the top ten; "Brown Sugar" was #13, one week away from jumping in.
11/5/77..."Star Wars Theme/Cantina Band" was #12, having just dropped out of the top ten; "How Deep Is Your Love" was #11, one week away from jumping in.
7/1/78..."You're the One That I Want" was #11, having just dropped out of the top ten; "Miss You" was #14, one week away from jumping in.
12/12/81..."Private Eyes" was #18, having just dropped out of the top ten; "I Can't Go for That" was #13, one week away from jumping in.
3/19/83..."Baby, Come to Me" was #21, having just dropped out of the top ten.
3/26/83..."Come On Eileen" was #11, one week away from jumping in.