For the above question, I am marked wrong for putting "le 1 mai", with the correct answer being "le 1er mai".
The lesson text implies to me that either are correct. It states that French dates require cardinal not ordinal numbers and includes "un (1)" in the list of examples of cardinal numbers. The "le premier (1er)" is then listed as an exception that "we do use". It is not clear from this whether "1er" must or may be used.
Could this please be clarified?