I think this only applies to the Sun square mantra and certain specific meditations.
For certain magical operations and meditations it is true that there are much more powerful astrological moments for this, such as the stellium in Leo especially if the Sun and/or Moon are involved, the days of Litha, Lammas and Ostara are stronger for Solar magic.
All solar mantras can also be used when the Sun is in Aquarius (Detriment) or Libra (fall) except the mantra of the planetary square of the Sun.
For example solar mantras ensure success during purification Sabbaths on any Saturday of the year or even better when starting with a Saturday of the new moon.
I do not recommend strengthening the Solar chakra when the Sun is in an unfavorable sign and waiting for a favorable day