From the Anglo-Saxon Chronicle: 1135

A.D. 1135. In this year went the King Henry over sea at the
Lammas; and the next day, as he lay asleep on ship, the day
darkened over all lands, and the sun was all as it were a three
night old moon, and the stars about him at midday. Men were very
much astonished and terrified, and said that a great event should
come hereafter. So it did; for that same year was the king dead,
the next day after St. Andrew's mass-day, in Normandy. Then was
there soon tribulation in the land; for every man that might,
soon robbed another. Then his sons and his friends took his
body, and brought it to England, and buried it at Reading. A
good man he was; and there was great dread of him. No man durst
do wrong with another in his time. Peace he made for man and
beast. Whoso bare his burthen of gold and silver, durst no man
say ought to him but good. Meanwhile was his nephew come to
England, Stephen de Blois. He came to London, and the people of
London received him, and sent after the Archbishop William
Curboil, and hallowed him to king on midwinter day. In this
king's time was all dissention, and evil, and rapine; for against
him rose soon the rich men who were traitors; and first of all
Baldwin de Redvers, who held Exeter against him. But the king
beset it; and afterwards Baldwin accorded. Then took the others,
and held their castles against him; and David, King of Scotland,
took to Wessington against him. Nevertheless their messengers
passed between them; and they came together, and were settled,
but it availed little.

Notes:
1st August is Lammas Day, "the feast of first fruits". A blessing was performed annually in the church on the 1st or 6th of August. From this we can reasonably conclude that Henry I left for Normandy in early August.

St. Andrew's day is 30th November, dating Henry's death to 1st December.

Midwinter day is usually around 21st - 22nd December

Wessington is a village in the county of Derbyshire, England