Vespers today marks the start of the proper Lenten Office. Anciently Lent began on the first Sunday with Ash Wednesday and the following three days added later. The Office on Ash Wednesday and the following days is really that of Septuagesima, with the same hymns, with the addition of the ferial preces.
Today, and for the rest of Lent with the exception of Sundays, Vespers are not sung at the usual time of late afternoon but sung before lunchtime. A rubric in the Spring volume of the Breviary, before first Vespers of the first Sunday of Lent states:
On this day, and thereafter until Holy Saturday, except on Sundays, Vespers are said before the principal meal at midday, even on Feasts.
This practice, much criticised by the reformers of the twentieth century Liturgical Movement, was popularly associated with the practice of fasting. However, a contrary view, and one I share, would be that the practice represents a symbolic inversion of time as a consequnce of the Fall, with the restoration of 'normality' with the victory of the LORD at Easter. The practice is not confined to the Roman (or other Western rites) as it is found in the East too. In the Slav-Byzantine typicon Vespers in the morning are prescribed not only in Lent but on many other penitential days too.
At Vespers the antiphons and psalms of Saturday are sung. The chapter Fratres: Hortamur vos, is proper and the Lenten hymn Audi, benigne Conditor is sung. The versicle and response, Angelis suis and Ut custodiant are used until Passiontide. The Suffrage of the Saints is sung after the collect of the day. At Compline the Dominical preces are sung.