The original settlers of the New Sweden colony were a mix of Swedes and Finns - Finland was a part of Sweden at the time. So, Finland was made an integral partner in the celebration of the 300th anniversary of New Sweden, in a supporting role in Delaware and a leading role in the commemoration events in Pennsylvania that followed.
1936 Landing of the Swedes - Delaware Tercentenary Half Dollar

New Sweden was more than just Fort Christina in Delaware, it also included settlements in parts of what is today (and was in 1938) part of eastern Pennsylvania and western New Jersey. Due to this, commemorative celebrations were held beyond Delaware in other parts of the Delaware River Valley (most notably in Chester, PA). The Governors of Pennsylvania and New Jersey recognized the importance of including Finland in the celebrations, and thus pushed for the Public Resolution signed by President Franklin Delano Roosevelt in 1936 to be amended to include enabling the President to formally invite Finland in addition to Sweden.
During consideration of the amendment in the House, Representative Samuel Davis McReynolds (D-TN) spoke in support:
"Last year we passed a bill at the request of the states of Delaware, New Jersey, and Pennsylvania, to invite the Swedish Government to participate in this celebration, the invitation stating "and other individuals."
The Historical Society of Pennsylvania, through this act, extended an invitation to the Finnish Minister to participate, and he went down to the State Department and they concluded the language was not quite broad enough, and that is what brought the matter up.
When the bill came over to the House from the Senate, we waited until at least two States had joined in the request, and they were the State of Pennsylvania and the State of New Jersey. They requested that the Finnish Government be invited. It would be very embarrassing if we did not do this, and I hope the Members of the House will pass this little amendment. I know it will give great satisfaction to them and to the people at large who so greatly admire the Finnish Government for keeping their obligations with the American people. [Applause.]"(Note: The "obligation" mentioned by McReynolds referred to the repayment of its 1919 food support loan made by the US to Finland following World War I. Finland was the only country to fully repay its WWI loan debt.)
The amendment was introduced in 1937, and though there was some debate in Congress over it, the amendment was passed and the Government of Finland received an official invitation to the Delaware celebrations.
In Delaware, Finland participated via an official delegation, attended a special service in Old Swedes Church, participated in the official monument dedication at Fort Christina State Park, had its Minister of Foreign Affairs, Dr. E. Rudolf W. Holsti address the gathered crowd at the dedication and attended official parties that were closed to the general public. All-in-all, Finland was a full participant in the Delaware tercentenary commemoration celebration, though it was partially in the shadow of Sweden.
On June 29, 1938 - just two days after the Delaware ceremonies - tercentenary celebrations were held in Chester, PA that marked the 300th anniversay of the "First Finnish Settlement in America" - Finland was front and center at the event. As part of the ceremonies, the Government of Finland gifted to America a monument to the early Finnish settlers. The brownish-red granite monument was sculpted by Valno Aaltonen, a noted Finnish sculptor; the granite was quarried in Paris, France from the same that produced the granite used for Napoleon's tomb.
Finnish Settlers Monument - Front
Finnish Settlers Monument - Back
(Image Credits: Both monument images are courtesy of the Historical Marker Database https://www.hmdb.org, the photographs were taken by Don Morfe.)For numismatic and philatelic collectors, Finland issued, respectively, a commemorative medal and a commemorative stamp.
1938 New Sweden Commemorative Medal of Finland

[b[1938 Finland Commemorative Postage Stamp

For details on these commemorative issues, see:
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1936 Delaware Tercentenary - Finnish CousinAs part of the ceremonies for the Finnish Monument, US President Roosevelt was presented with a gold version of the commemorative medal.
For other of my posts about commemorative coins and medals, including more Delaware half dollar stories and its Swedish cousins, see:
Commems Collection