The author has come to understand that today, in the UK is the day hairdressers are permitted to open again.

To that end, let us analyse the astrology used to find auspicious times to cut hair
https://books.google.com/books?id=EyZlAAAAcAAJ&pg=RA1-PA133#v=onepage&q&f=false

This passage above comes from William Ramesey's Astrologia Restaurata, which is one of the most comprehensive treatises on the subject that is available for free, as yet.

Lets unpick this rather dense passage

The main factor in this election- indeed, all electional astrology, is the position and condition of the moon. Ramesey first tells us that it is best to cut the hair when the Moon is in a "Common sign" i.e.
Gemini, Sagittarius, Virgo and Pisces

In addition to that, he states the Moon must be well dignified. By this is meant that the moon should not be subject to astrological afflictions- i.e. square or in opposition to Saturn or Mars, combust, or in the signs of her debility (i.e. Capricorn or Scorpio)

The latter forms a qualification to Ramesey's next statement, saying that the Moon should be in an Earthy sign- Capricorn is an earth sign, but as the Moon is debilitated in Capricorn , and hence it should be avoided.

From this point on, things get a little complicated. It seems people in Ramesey's time believed that depending on the time you cut your hair, the hair would either grow back quickly or slowly.

Ramesey taught you how to determine the hair-cutting times to create either effect

Let us assume you wanted the hair to grow back slowly (natural- barbers are hard to come by) Then, we are told, you should cut the hair in a "barren" sign.
This is the name that was collectively used to refer to Gemini, Leo and Virgo. (and sometimes Aries) https://www.astro.com/astrowiki/en/Barren_sign

... this is because all three signs represent creatures that reproduce slowly (Lions, young boys) or not at all (virgin), although there are other explanations involving the planetary rulers of the signs https://www.astro.com/astrowiki/en/Barren_sign

Conversely, the "fruitful" signs are Cancer, Scorpio & Pisces. The 3 signs represent animals that spawn masses of offspring.

Naturally, cutting hair when the Moon is in these would cause the hair to grow back thickly-- but one has doubts about scorpio, as it is the Moon's fall

it should be noted that at time of writing, the Moon is in Aries, and another early modern book on electional tells us cutting hair when the moon in this sign "threatens baldness" https://books.google.com/books?id=RSDVW69UeN4C&dq=mikropanastron&pg=PA261#v=snippet&q=cutting&f=false

Returning to Ramesey, he next gives other conditions for optimal hair-cutting, namely its relation to the other planets.

The first is the Moon's phase. We are told that if the hair is cut while the Moon is "increasing in light" it would grow back quickly.

"Increasing in light" means the moon should be waxing- i.e. any time between the new moon and the full moon

Conversely, if the moon was decreasing in light-- i.e. waning, or after the full moon-- the hair would grow back but slowly

Curiously enough, we are also told that cutting hair on the opposition of the moon and sun-i.e. the full moon- would make it grow back thick.

In turn, it seems cutting the hair when the moon is "afflicted" by Saturn (i.e. square, opposition, or conjoined Saturn) has a contradictory effect. On the one hand, doing so would make the hair grow back slowly, but on the other, it would cause dandruff...

In turn, it is not amiss to list this thread by @avtansa wherein a long-standing taboo of hair-cutting on Tuesdays in India is discussed https://twitter.com/avtansa/status/1243301946435477507

See also this response on why Tuesday, thursday and Saturday are tabooed - They are the days ruled by the Malefics (Tue, Sat), and cutting hair in diei Jovis (thu) would risk damaging the bounty of the greater benefic... https://twitter.com/Atmaprajnananda/status/1243494031461625857

Readers would also recall Ramesey warned against the Moon making aspects to Mars/ Saturn when hair was cut