The "143 day" is often associated with a symbolic way of expressing "I love you." The numbers 1, 4, and 3 represent the number of letters in each word of the phrase:

• 1 = I
• 4 = love
• 3 = you

So, "143" is a numeric code for saying "I love you." In some contexts, "143 day" might be a day dedicated to celebrating love or expressing affection, but this is less common and not an official holiday. If you meant something else by "143 day," please provide more details!