Acts 2:1

And when the day of Pentecost came [Note: This was a Jewish festival held fifty days following the Passover Festival. Lev. 23:15-21], they [i.e., the twelve apostles] were all gathered together in one place [i.e., their second story living quarters, 1:13 ?].

1 Corinthians 16:8

But I will wait [here] in Ephesus until the Day of Pentecost [Note: This was an annual Jewish festival, held at spring harvest time];

Hebrews 2:1

So [then], we [Christians] should pay closer attention to the things we have heard, so that we do not drift away from [believing and practicing] them.

Treasury of Scripture Knowledge

Summary

Seven weeks shalt thou number unto thee: begin to number the seven weeks from such time as thou beginnest to put the sickle to the corn.

Bible References

General references

Acts 2:1
And when the day of Pentecost came [Note: This was a Jewish festival held fifty days following the Passover Festival. Lev. 23:15-21], they [i.e., the twelve apostles] were all gathered together in one place [i.e., their second story living quarters, 1:13 ?].
1 Corinthians 16:8
But I will wait [here] in Ephesus until the Day of Pentecost [Note: This was an annual Jewish festival, held at spring harvest time];
Hebrews 2:1
So [then], we [Christians] should pay closer attention to the things we have heard, so that we do not drift away from [believing and practicing] them.