Secretaries day in Belgium

Administrative Professionals Day (also known as Secretaries Day or Admin Day) is an observance but not a public holiday in the United States, celebrated annually on the Wednesday during Administrative Professionals Week, the last full week of April. The secular holiday recognizes the work of secretaries, administrative assistants, receptionists, and other administrative support professionals. Typically administrative professionals are given cards, flowers, chocolates, and lunches

Observation by country
In Hong Kong, it is celebrated annually on the Wednesday of the last full week of April.
In Malaysia, Administrative Professionals Week is always held annually during the last full week in April. Administrative Professionals Day is always held the Wednesday of that week.
In New Zealand, it is on the third Wednesday of April.
In South Africa, it is celebrated annually on the first Wednesday of September.
In Australia, it is celebrated annually on the first Friday of May.
In United Kingdom, it is celebrated on Twitter.
In Ireland, it is celebrated in the offices of top professional firms. The inaugural day for this celebration being 27 April 2016.

Most countries do not follow this patronising attitude.

It's always the third Thursday in April - So this year it's April 20th.

In French it's known as "fêtes des secrétaires". Don't know what the equivalent (if any) is in Flemish.