Question: On what date did the German offensive of which the First Battle of Villers-Bretonneux was a part begin?

Context paragraphs:
Winter operations 1914–1915: Winter operations 1914–1915 is the name given to military operations during the First World War from 23 November 1914 – 6 February 1915, on the part of the Western Front held by the British Expeditionary Force (BEF), in French and Belgian Flanders.  Both sides had tried to advance in Flanders after the northern flank had disappeared during the Race to the Sea in late 1914.  Franco-British attacks towards Lille in October were followed by attacks by the BEF, Belgians and a new French Eighth Army (Général Victor d'Urbal).  A German offensive began on 21 October but the 4th Army ("Generaloberst" Albrecht, Duke of Württemberg) and 6th Army ("Generaloberst" Rupprecht, Crown Prince of Bavaria) were only able to take small amounts of ground, at great cost to both sides, at the Battle of the Yser (16–31 October) and further south in the First Battle of Ypres (19 October – 22 November).

Battle of Kursk: The Battle of Kursk was a Second World War engagement between German and Soviet forces on the Eastern Front near Kursk (450 km south-west of Moscow) in the Soviet Union during July and August 1943.  The battle began with the launch of the German offensive, Operation Citadel (German: "Unternehmen Zitadelle" ), on 5 July, which had the objective of pinching off the Kursk salient with attacks on the base of the salient from north and south simultaneously.  After the German offensive stalled on the northern side of the salient, on 12 July the Soviets commenced their Kursk Strategic Offensive Operation with the launch of Operation Kutuzov (Russian: Кутузов ) against the rear of the German forces in the northern side.  On the southern side, the Soviets also launched powerful counterattacks the same day, one of which led to a large armoured clash, the Battle of Prokhorovka.  On 3 August, the Soviets began the second phase of the Kursk Strategic Offensive Operation with the launch of Operation Polkovodets Rumyantsev (Russian: Полководец Румянцев ) against the German forces in the southern side of the Kursk salient.

Operation Citadel: Operation Citadel (German: "Unternehmen Zitadelle" ) was a German offensive operation against Soviet forces in the Kursk salient during the Second World War on the Eastern Front that initiated the Battle of Kursk.  The deliberate defensive operation that the Soviets implemented to repel the German offensive is referred to as the Kursk Strategic Defensive Operation.  The German offensive was countered by two Soviet counter-offensives, Operation Polkovodets Rumyantsev (Russian: Полководец Румянцев ) and Operation Kutuzov (Russian: Кутузов ).  For the Germans, the battle was the final strategic offensive that they were able to launch on the Eastern Front.  As the Allied invasion of Sicily began Adolf Hitler was forced to divert troops training in France to meet the Allied threats in the Mediterranean, rather than use them as a strategic reserve for the Eastern Front.  Germany's extensive loss of men and tanks ensured that the victorious Soviet Red Army enjoyed the strategic initiative for the remainder of the war.

First Battle of Villers-Bretonneux: The First Battle of Villers-Bretonneux (30 March – 5 April 1918), took place during Operation Michael, part of the German Spring Offensive on the Western Front.  The offensive began against the British Fifth Army and the Third Army on the Somme and pushed back the British and French reinforcements on the north side of the Somme.  The capture of Villers-Bretonneux, close to Amiens, a strategically-important road- and rail-junction, would have brought the Germans within artillery-range.  In late March, Australian troops were brought south from Belgium as reinforcements to help shore up the line and in early April the Germans launched an attack to capture Villers-Bretonneux.  After a determined defence by British and Australian troops, the attackers were close to success until a counter-attack by the 9th Australian Infantry Brigade and British troops late in the afternoon of 4 April restored the situation and halted the German advance on Amiens.

First Battle of the Masurian Lakes: The First Battle of the Masurian Lakes was a German offensive in the Eastern Front during the early stages of World War I.  It pushed the Russian First Army back across its entire front, eventually ejecting it from Germany.  Further progress was hampered by the arrival of the Russian Tenth Army on the Germans' left flank.

First Battle of Champagne: The First Battle of Champagne (French: "1ère Bataille de Champagne" ) was fought from 20 December 1914 – 17 March 1915 in World War I in the Champagne region of France and was the second offensive by the Allies against the Germans since mobile warfare had ended after the First Battle of Ypres in Flanders(19 October – 22 November 1914).  The battle was fought by the French Fourth Army and the German 3rd Army.  The offensive was part of a strategy by the French army to attack the Noyon Salient, a large bulge in the new Western Front, which ran from Switzerland to the North Sea.  The First Battle of Artois began on the northern flank of the salient on 17 December and the offensive against the southern flank in Champagne began three days later.

Battle of the Lys (1918): The Battle of the Lys (7–29 April 1918) , also known as the Lys Offensive, the Fourth Battle of Ypres, the Fourth Battle of Flanders and Operation Georgette (Portuguese: "Batalha de La Lys" and French: "3ème Bataille des Flandres" ), was part of the 1918 German offensive in Flanders during World War I, also known as the Spring Offensive.  It was originally planned by General Ludendorff as Operation George but was reduced to Operation Georgette, with the objective of capturing Ypres, forcing the British forces back to the channel ports and out of the war.  In planning, execution and effects, Georgette was similar to (although smaller than) Operation Michael, earlier in the Spring Offensive.

Battle of the Avre: The Battle of the Avre (4–5 April 1918), part of the First Battle of Villers-Bretonneux, constituted the final German attack towards Amiens in World War I.  It was the point at which the Germans got the closest to Amiens.  It was fought between attacking German troops and defending Australian and British troops.  The attack was an attempt to take Amiens, where other aspects of Operation Michael had failed.  The Avre marked the beginning of the end for Ludendorf's March Offensive.

Spring Offensive: The 1918 Spring Offensive or Kaiserschlacht ("Kaiser's Battle"), also known as the Ludendorff Offensive, was a series of German attacks along the Western Front during the First World War, beginning on 21 March 1918, which marked the deepest advances by either side since 1914.  The Germans had realised that their only remaining chance of victory was to defeat the Allies before the overwhelming human and matériel resources of the United States could be fully deployed.  They also had the temporary advantage in numbers afforded by the nearly 50 divisions freed by the Russian surrender (the Treaty of Brest-Litovsk).

Hangard Wood: Hangard Wood is a locality south of Villers-Bretonneux northern France.  It was the site of Hangard village and a battle in World War I.  The battle of Hangard Wood was part of the German offensive Operation Michael, in the Arras - St-Quentin-La Fére sector of the Somme fought in March 1918.  The battle of Hangard Wood was more specifically part of the larger second battle of Villers-Bretonneux, fought between Canadian British/Australian/French and German armies.

Supporting paragraphs: First Battle of Villers-Bretonneux Spring Offensive
Answer: 21 March 1918