Acts 1

During the mortal ministry of the Savior, there are many occasions where the Holy Ghost was present and bore testimony of the Savior.  But as the Savior was about to leave -

"...being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me.
5 For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence."  (New Testament | Acts 1:4 - 5)

It appears that the fullness of the Gift of the Holy Ghost had not yet been bestowed upon these brethren for -

"Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen..."  (New Testament | Acts 1:2)

It appears that by this Gift that the Lord would continue to direct His work on the earth for -

"Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen..."  (New Testament | Acts 1:2)

Many claim that the day of Pentecost was when the Gift of Holy Ghost fell upon them.  That may be so, but I believe that Peter received the Holy Ghost where under his administration -

"...they gave forth their lots; and the lot fell upon Matthias; and he was numbered with the eleven apostles."  (New Testament | Acts 1:26)

I personally believe that when they gave forth their lots, these lots were the brethren's recommendations.  I believe that Peter chose Matthias by the Spirit or in other words, the lot fell on Matthias.