Also known as the Golden Ratio and purported to show up in, well, everything that anybody is willing to measure and then generously round the numbers in the correct direction, φ is equal to 1.618039887…
Finding a day for π is a piece of cake. 3.14 is March 14. Easy. Finding a day for φ, rounded off to 1.62, is a wee bit challenging. Last I checked, however, there had not been 62 days in January since Julius Caesar reformed the calendar. And, there is no month 16.
Fortunately, European dating provides a solution. Here, we do dates in a sensible hierarchical fashion reflecting the insensible social hierarchies inherited from before WWI. Instead of MONTH DAY, YEAR, it is DAY MONTH YEAR. In this notation, φ=1.62 renders as 16 February.
So, HAPPY PHI DAY! to you and yours.