Context #1

Jay: I can’t believe mom bought us a trip to Hawaii. Maybe she’ll
get us a car too.
Saul: Calm down Jay. One
swallow does not make a summer. Remember that mom has money
limitations. Let’s not get ahead of
ourselves.
Jay: You don’t think
mom would do that for us? 
Saul: Be realistic. Just
because mom got a new job, doesn’t mean she can afford everything.

Context #2

Peter: I got into college in NY. I’m not worried anymore. I’m sure
all the other colleges will accept me too.
Sonia: One swallow does not make a summer buddy. Every school has different
standards and rules. Let’s not get too confident about what schools will accept
you yet. You should wait before making a decision.
Peter: But this was the
most difficult school to get into. How can the other schools not accept me?
Sonia: Sometimes school departments
make a difference. It’s not necessarily the entire university that makes that
decision. 

Meaning:The expression
“one swallow does not make a summer”
means that just because one good thing happened, you cannot expect every other
thing that will happen to be good. For example, some people think that if
something good happens at the beginning of a trip, it’s a sign that everything
else will go well. In reality, that is not true. In the first example, Jay
thinks that because the mother bought them a trip, surely she will also buy
them a car, but there’s no evidence to prove this. It’s just a feeling. In the
second example, Peter thinks every college he applied to will accept him
because he got into the most difficult one. However, that’s just his belief,
not a fact. He is not aware that most universities have differences in
acceptance policies and that the standards of each department in that college
vary. 

https://languagesystems.edu/