You say one swallow doesn't make a summer to mean that although something good has happened, the situation may not continue to be good, and you cannot rely on it. Sales in December were up about 1 percent, which of course, is good news. One swallow, however, doesn't make a summer and it remains to be seen whether they can maintain this growth in the difficult months ahead. Note: People often use the more literary phrase one swallow does not a summer make. One swallow does not a summer make, but a visit to the Hotel Metropole in Ipswich goes a long way to improving that city's reputation. Note: Swallows are a type of bird. The reference here is to the arrival of swallows in Europe at the beginning of summer, after spending the winter further south.