The Collaborative International Dictionary of English v.0.48:

Shrovetide \Shrove"tide`\, n. [From shrive to take a confession
   (OE. imp. shrof, AS. scr[=a]f) + tide.]
   The days immediately preceding Ash Widnesday, especially the
   period between the evening before Quinguagesima Sunday and
   the morning of Ash Wednesday.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Shrovetide
    n 1: immediately preceding Lent