An API-only service that allows you to determine whether Mercury is in retrograde for a given date.

Seems essential to me.

Mercury in retrograde dates for the next decade.

2021
January 30 – February 20 in air-sign Aquarius
May 29 – June 22 in air-sign Gemini
September 27 – October 18 in air-sign Libra

2022
January 14 – February 3 starts in air sign Aquarius, ends in earth-sign Capricorn
May 10 – June 2 starts in air sign Gemini, ends in earth-sign Taurus
September 9 – October 2 starts in air-sign Libra, ends in earth-sign
December 29 – January 18 in earth-sign Capricorn

2023
See above, Dec 29, 2022 – Jan 18, 2023- goes direct in earth sign Capricorn
April 21 – May 14 in earth sign Taurus
August 23 – September 15 in earth sign Virgo
December 13, 2023 – January 1, 2024 in fire-sign Sagittarius

2024
See above, January 1, 2024 goes direct in fire-sign Sagittarius
April 1- April 25 in fire-sign Aries
August 4 – August 28 in earth-sign and ends in fire-sign Leo
November 25 – December 15 in fire-sign Sagittarius

2025
March 14 – April 7 starts in fire-sign Aries, ends in water-sign Pisces
July 17 – August 11 in fire-sign Leo
November 9 – November 29 starts in fire-sign Sagittarius, ends in water-sign Scorpio

2026
February 25 – March 20 in water-sign Pisces
June 29 – July 23 in water-sign Cancer
October 24 – November 13 in water-sign Scorpio

2027
February 9 – March 3 starts in water-sign Pisces, ends in air-sign Aquarius
June 10 – July 4 starts in water-sign Cancer, ends in air-sign Gemini
October 7 – October 28 starts in water-sign Scorpio, ends in air-sign Libra

2028
January 24 – February 14 in air-sign Aquarius
May 21 – June 13 in air-sign Gemini
September 19 – October 11 in air-sign Libra

2029
January 7 – January 27 starts in air-sign Aquarius, ends in earth-sign Capricorn
May 1 – May 25 in earth-sign Taurus
September 2 – September 24 starts in air-sign Libra, ends in earth-sign Virgo
December 21 – January 10, 2030 in earth-sign Capricorn

2030
See above, December 21, 2029 – January 10, 2030 in earth-sign Capricorn
April 12 – May 6 starts in earth-sign Taurus, ends in fire-sign Aries
August 15 – September 8 in earth-sign Virgo
December 5 – December 25 starts in earth-sign Capricorn, ends in fire-sign Sagittarius