In the Divino Afflatu calendar, the St. Mark feast is transfered to 28th of April, since the original date, April 25th, was within the Easter Octave this year.
On the same day as St. Mark's feast, there is a not so well-known penitential day called Major Rogation. This is probably the reason that your calendar says it is an abstinence day. I guess that the Major Rogation was moved the same way as the feast of St. Mark.