From my father's notes: The superior quality of 15 Av over the fifteenth of every other month is this: The fifteenth represents the visible completeness (of the moon), but (the moon) is still only a recipient (of light). However, on 15 Av the light is firmly fixed in it, for the ascent is in reverse proportion to the drastic descent of Tisha B'Av...

This then is the meaning of "The sun's power is weakened" (on 15 Av after its summer intensity). The idolators calculate their calendar by the sun of evil, which on the 15th is weakened... In the days of the Sanctuary they (the nations) were subservient to Shlomo (King Solomon); certainly it shall be so in the Time-to-Come when "(the nations) will flow to Him" in a state of total bitul (nullification), and "I will remove the spirit of impurity etc." A token of this is expressed on the 15th of Av when the sun is weakened...