Question: The battle where Bodo Sandberg earned the Cross of Merit and the Airman's Cross for his bravery started on what date?

Context articles: Bodo Sandberg
List of recipients of the Hanseatic Cross
German Cross
Lower Saxony Order of Merit
Battle of the Netherlands
Military Merit Cross (Prussia)
Carl Eduard War Cross
Military Merit Cross (Bavaria)
August Schmidt (Luftwaffe)
Military Order of William

Bodo Sandberg (September 23, 1914 - May 2, 2005) was a fighter pilot in the Royal Netherlands Air Force and 'Engelandvaarder' during World War II.
 He was awarded the Cross of Merit and the Airman's Cross for his bravery during the German invasion of May 1940.
The Hanseatic Cross ("Hanseatenkreuz") was a decoration of the three Hanseatic city-states of Bremen, Hamburg and Lübeck, who were members of the German Empire during World War I.
 Each republic established its own version of the cross, but the design and award criteria were similar for each.
 The cross was awarded for merit in war, and could be awarded to civilians as well as military personnel.
 When awarded for bravery or combat merit, it was the three cities' equivalent of the Prussian Iron Cross.
The German Cross (German: "Deutsches Kreuz" ) was instituted by Adolf Hitler on 28 September 1941.
 It was awarded in two divisions: gold for repeated acts of bravery or achievement in combat; and silver for distinguished non-combat war service.
 The German Cross in Gold ranked higher than the Iron Cross First Class but below the Knight's Cross of the Iron Cross, while the German Cross in Silver ranked higher than the War Merit Cross First Class with Swords but below the Knight's Cross of the War Merit Cross with Swords.
The Lower Saxony Order of Merit (German: "Niedersächsischer Verdienstorden" ) is a civil order of merit, of the German State of Lower Saxony.
 The order was established 27 March 1961.
 The order is presented in three classes, the highest is the Grand Cross of Merit (Großes Verdienstkreuz), the next is the Cross of Merit First Class (Verdienstkreuz 1.
 Klasse), and the lowest is the Cross of Merit on Ribbon (Verdienstkreuz am Bande).
The Battle of the Netherlands (Dutch: "Slag om Nederland" ) was part of Case Yellow (German: "Fall Gelb" ), the German invasion of the Low Countries (Belgium, Luxembourg, and the Netherlands) and France during World War II.
 The battle lasted from 10 May 1940 until the surrender of the main Dutch forces 14 May.
 Dutch troops in the province of Zeeland continued to resist the "Wehrmacht" until 17 May when Germany completed its occupation of the whole nation.
The Military Merit Cross ("Militär-Verdienstkreuz") was the highest bravery award of the Kingdom of Prussia for non-commissioned officers and enlisted soldiers.
 It was also known as the Golden Military Merit Cross ("Goldenes Militär-Verdienstkreuz") to distinguish it from the Military Decoration 1st Class ("Militär-Ehrenzeichen I. Klasse"), a lesser Prussian enlisted bravery decoration which was an identical cross but in silver.
 The Military Merit Cross came to also be known as the "Pour le Mérite for non-commissioned officers and enlisted men" ("Orden Pour le Mérite für Unteroffiziere und Mannschaften"), after the Pour le Mérite, Prussia's highest military decoration for officers.
The Carl Eduard War Cross (German: "Carl-Eduard-Kriegskreuz" ) was a military decoration of Saxe-Coburg and Gotha presented during World War I.
 Established 19 July 1916, by Charles Edward, Duke of Saxe-Coburg and Gotha, the cross recognized military merit and bravery in battle, without regard to rank.
 Awarded only 97 times, it is one of the rarest of World War I German military decorations.
 Recipients must have already been holders of the Iron Cross, 1st Class and been serving in the "6 Thüringischen Infanterie-Regiment Nr.
 95".
The Bavarian Military Merit Cross ("Militär-Verdienstkreuz") was that kingdom's main decoration for bravery and military merit for enlisted soldiers.
 It was intended "to reward extraordinary merit by non-commissioned officers, soldiers, and lower-ranking officials."
 It was originally established on July 19, 1866 as the 5th Class of the Military Merit Order, which was the main decoration for bravery and military merit for officers and higher-ranking officials.
 Civilians acting in support of the army were also made eligible for the decoration.
August Schmidt (1 February 1883 – 23 November 1955) was a highly decorated General der Flakartillerie in the Luftwaffe during World War II.
 He was also a recipient of the Knight's Cross of the Iron Cross.
 The Knight's Cross of the Iron Cross was awarded to recognise extreme battlefield bravery or successful military leadership.
 August Schmidt was captured by British troops on 8 May 1945.
 In October 1947 he was sentenced to life imprisonment because of war crimes committed against captured British airman.
 His sentence was later reduced to ten years, and in 1950 he was released due to health reasons.
The Military William Order, or often named Military Order of William (Dutch: Militaire Willems-Orde, abbreviation: MWO), is the oldest and highest honour of the Kingdom of the Netherlands.
 The Order's motto is "Voor Moed, Beleid en Trouw" (For Bravery, Leadership and Loyalty).
 The chivalric order was established on 30 April 1815 by King William I and was presented for feats of excellent bravery on the battlefield and as a meritorious decoration to senior military officers.
 Comparable with the French Légion d’Honneur but far less often awarded, the Military William Order is a chivalry order of merit open to everyone regardless of rank and nobility, and not only to Dutch military but also foreigners.
 To date membership of the Order is extremely rarely awarded and only for excellent bravery in battle.