Columbia subject headings:
May Day (Labor holiday)--Historiography--Congresses.
Socialism--Historiography--Congresses.
Harvard subject headings:
May Day (Labor holiday)--Historiography--Congresses.
Socialism--Historiography--Congresses.
Princeton subject headings:
May Day (Labor holiday)--Historiography--Congresses.
May Day--Historiography--Congresses.
Socialism--Historiography--Congresses.