John Gill Commentary Deuteronomy 16:10

"And thou shalt keep the feast of weeks unto Jehovah thy God with a tribute of a freewill-offering of thy hand, which thou shalt give, according as Jehovah thy God blesseth thee:" — Deuteronomy 16:10 (ASV)

And you shall keep the feast of weeks unto the Lord your God ,
&c.] The feast of Pentecost, at which time the Spirit was poured down upon the apostles, (Acts 2:1)

with a tribute of a freewill offering of your hand ;
there were two wave loaves which were ordered to be brought and seven lambs, one young bullock and two rams for a burnt offering, together with the meat and drink offerings belonging thereto, and a kid of the goats for a sin offering, and two lambs for a peace offering, (Leviticus 23:17–19) .

And besides all this, there was to be a voluntary contribution brought in their hands; for this was one of those feasts at which all the males were to appear before the Lord, and none of them empty:

which you shall give
unto the Lord your God,

according as the Lord your God has blessed you ;
no certain rate was fixed, it was to be a free gift, and in proportion to a man's abilities, or what the Lord had blessed him with.