| Ad diem illum laetissimum Latin for 'On that most happy day' Encyclical of Pope Pius X | |
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| Signature date | 2 February 1904 |
| Subject | Immaculate Conception |
| Number | 2 of 17 of the pontificate |
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Ad diem illum laetissimum is an encyclical of Pope Pius X on the Immaculate Conception, dated 2 February 1904, in the first year of his Pontificate. It is issued in commemoration of the fiftieth anniversary of the dogma of the Immaculate Conception. The first reason for Pius to write the encyclical was his desire to restore of all things in Christ, which he had defined as his motto in his first encyclical letter. It explains the Mariology of Pius X.