Fantastic Four (1961) #35 and #36

Cover by Jack Kirby

Cover Date: February 1965

On-Sale Date: December 1964

Synopsis: Professor X and Cyclops go to State University to look for mutants, and bump into the Fantastic Four.

CHRONOLOGICAL NOTES:

Page 2, panels 4 and 5

Xavier and Cyclops make a rather brief cameo here, having just finished interviewing some students to determine if they are mutants. Note that although Reed Richards has heard of Xavier's school, the two have not met before.

Elsewhere in this story, Reed Richards proposes to Sue Storm, which becomes relevant for the very next issue.

COUNTING THE DAYS

For the X-Men, this takes place over a single day. It has to be during the school year, so it's probably in late August or September.

Cover by Jack Kirby

Cover Date: March 1965

On-Sale Date: January 1965

Synopsis: The X-Men and Professor X attend the engagement party of Reed Richards and Sue Storm.

CHRONOLOGICAL NOTES:

Page 8, panel 2

The X-Men can be seen here being the life of the party at Reed and Sue's engagement.  (Well, not Cyclops, but what do you expect.)  Professor X is still maintaining his secret identity, as the X-Men must pretend that they don't know him.  Amusingly, the Thing met Professor X last issue and has already forgotten who he is.

After the party ends the FF are attacked by the Frightful Four, but the X-Men have left by that point.

COUNTING THE DAYS

For the X-Men this takes place over a single day, not too long after their guest appearance in Fantastic Four (1961) #35.  In publishing terms, both of these issues came out between X-Men (1963) #9 and #10.