wax and wane - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Increase and decrease, as in size, number, strength, or intensity, as in Enrollments in these programs wax and wane from year to year. This expression alludes to the phases of the moon, with its periodic changes in size. It was first recorded in the 1300s.